This resolves #2189.

> build isolation would be a bit difficult to use in our case, as for instance when building scipy we need the patched numpy on the host and not the numpy version specified in pyproject.toml (which would be unpatched)

This is indeed the case, certain packages cannot be isolated. My strategy is to make a list of packages that shouldn't be isolated and add symlinks from the isolated build environment into the `.artifacts` directory to "unisolate" them. Then we remove the unisolated package requirements from the list of packages to install, in case pesky constraints aren't satisfied. In particular, packages that expect to be used with `pypa/build` often feel free to put very specific constraints on their build dependencies (often asking them to be == to a particular version). Specific version constraints is good for build reproducibility and with build isolation doesn't cost anything. So we just ignore the constraints. Hopefully nothing goes wrong.

In particular, any package that does stuff both at build time and at runtime and requires synchronization between the build time and run time environments needs the unisolation. This includes cffi with `_cffi_backend.so`, and of course numpy and scipy. pycparser needs to be unisolated because it is a dependency of cffi.

Currently I have also unisolated pythran and cython, though these are build time only tools and do not really need to be unisolated. Cython I unisolated specifically because numcodecs needs it but it isn't in the numcodecs build dependencies. Pythran I unisolated because of a problem with the scipy build which I don't fully understand (some problem with long double feature detection). I believe both of these are removable, particularly once #2132 is finished.
Use pypa/build. This resolves #2189.

> build isolation would be a bit difficult to use in our case, as for instance when building scipy we need the patched numpy on the host and not the numpy version specified in pyproject.toml (which would be unpatched)

This is indeed the case, certain packages cannot be isolated. My strategy is to make a list of packages that shouldn't be isolated and add symlinks from the isolated build environment into the `.artifacts` directory to "unisolate" them. Then we remove the unisolated package requirements from the list of packages to install, in case pesky constraints aren't satisfied. In particular, packages that expect to be used with `pypa/build` often feel free to put very specific constraints on their build dependencies (often asking them to be == to a particular version). Specific version constraints is good for build reproducibility and with build isolation doesn't cost anything. So we just ignore the constraints. Hopefully nothing goes wrong.

In particular, any package that does stuff both at build time and at runtime and requires synchronization between the build time and run time environments needs the unisolation. This includes cffi with `_cffi_backend.so`, and of course numpy and scipy. pycparser needs to be unisolated because it is a dependency of cffi.

Currently I have also unisolated pythran and cython, though these are build time only tools and do not really need to be unisolated. Cython I unisolated specifically because numcodecs needs it but it isn't in the numcodecs build dependencies. Pythran I unisolated because of a problem with the scipy build which I don't fully understand (some problem with long double feature detection). I believe both of these are removable, particularly once #2132 is finished.
PNG


IHDRiCCPICC profile(}=H@_SZv(␡:Yq*BZu04$).kŪ "%/)=B4ktL%b&*^!A""3˘$:{zY9ԜH<& ޴
aVUs1.Hu76өy0XhcYԈSXY+WYuH`K BA%a#FN;\D.\%0r,
Z	/)_c|;N?Wz_3ZZ.[\'C6eWy)5>N4uewxwO{oiry]bKGD	pHYs+tIME
DwIDATxypU׹7}PDJ[HvJm8b]F\:TFEjԱ-uCb_h"` ,!='%jM|d|g
|>1\x/@ x/@ x/@ x/@ x/@ x/R8E7t6l4?ϟqƵk]{rg}w}W3fh׺;M6?Tnn+uپk2dF뮻N3fP;Ǐk̘1?,LVZZF9sh֬Y}뮻O|ꫯBxYkȐ!6m͛;Trrr4h 3C0@/i$QF^zx69s7T^voٲMmۼfر!q>7lrrr}z5insĉ.ceY檫2;vнԓ-_yyyGBI	_J]vviO>DW\q-Z?<$Zׯѣן:mذAcƌ}ݧ.NEڹs&O믿^_~%a^8p@?n:F^|EM<Y~iHn_MMVX)S(??OzW_+6mڤ'j׮]



;t]wq0Xln喐S>'}|駺u!. jԩzW9;{9ek֬SO=գ9''GsQaaa9OgΜܹsޤ^/@߱l2m޼q۷{~I-XO~}xPWW 0,zS*<<1F:ydP5m[oUfff
dɒS\\F$\.ۦ<=z4ڵk}Y76L^xaǣ<eff;wԟ'qv+m[Reee{sGEDDBU'KVVV۸w^3}YbE%_]Òάkرf۶m}*..6W6/>>u]CIIyLLLL}NNNxud^zi:oQFuڵCÒ>Kq'N֭[n:EEEmjժznƌ
^vv>~۸n=4}t?B%&&jҥڿjmOk8!!Awy<)Sm[TTիW{>""Bsվ}twȲ,~a~Q!teY>]YYw}֬YTVVqv>|iŊZre94##C۷oׄ	{Wda+.Tg?<ߢf{~ر#>^6g-^o^{?KNNjܹ}ܾ}nFٶC@RR7~[o}v0ˊlMNNkaۭ_~YkѣGCfrPJ7xxcՊmu-[#3K/
]viѢEzݲo4F'x5qo^xAn;d_0oٲW]cƌ
O9^;4{VWWWcdXm޼Y׿i&z}4{jViucǎرcC.Z^wǏsc<x0 }.?u7~m۶iРA׿~z=Cm4clxbZ[;\tE~S3o)Ƿ]}eY7nE ^Α#Gj~K"\R<LKyyf͚'Ol;w\.SRRBwzu<p6_߮V$jP^vez뭷5{+t~i޼yڿW^y	
[(eWo@3eo81F7tmz\;v/;,r/'?њ5kx<Zp۩7nСںu8i/#lWUUkV˗/ן풒uVs^裏5YQQfΜ֬Yp_əe;>^AYn.\]vvfΜ)7nܨKl^{-xw\k&`?\sUMMMޣ-SK5k'УqsY3Add6mڤɓ'k߾}~6MAo5g}֦)_pȑ#CrV\믿/>}>-[?N8o%Kh~;vL^{mPS?Nk/m+//GD|iii^A/|V9sF3gz+Wrr^裏gϞVViīV);s`@qY-^X1jh6lhVӁ6) }@CCNeeem7o޼N{϶LAxbmݺU^W7pSw!M4I/v={vw[ ^p!S$VĉAձv6c***t	{ڰanŊ
۔{ojmڴ)2:thHn[LL'^}{1:i$g???ߡ0e0Z."@hƍ,KgڵAMAܚpL h[|˟$25M<sLN 8ڼyMS_zmzSdڽ{Nz߻i
⌌3e0Masʕ:p.nێ` f`@P&N'xBYYYzԯ_nߦ#GjѢE~%Ct-5nWSZZ4a};	chd9'3yސm[gL>1@=S@~vcn[sW/ۭ$	^3߿?$x<.OKKu*33쁽q;.OMM ,;]tW_}P-g}6l9>Cr/HSL[nm!G~;mM֫ѣGOm3c.Fuuu9^_~V׫nIIJJҏ~^s~sG-_<_&]zӧ|vSNiڻwv,/ԬY{n~ӟ}}0f? L:mg߿-[MCCq]~ھ}yT=.622R=P^˵n:7N׿ׯ_w3yVX/% D0,#ayn]C>3jôiѣG+11
x<Sffz͝wީÇw}|M}󕙙toҥK5dȐnF?ٳgw{۶b=zTAfҤIZ`Aڰa
/gZf.\wLoG;legg6vX=ݺ
ڲeK8^z>]}}=R(ip^͟?_=X8;}jғDmڴI\A,K_5vX@З-_\7|shy[o.`Cu?A`^^|E==b#FO>ԩS9rvޭ5QQQz7tR@e˖i!krt7jϞ=}O.K7|ӧz;kݻ5o<xh
u?eY
ok<
A?3t!_^Vɓ'n͚5K+V3㨆i̙z't%p.#<B讟WtzD[mkZuu޵caK,ђ%Ko>裏\tiKMMUZZFٳgkΜ9JJJiٳgSbb"7h+&t]s5Z`@O9_?꿵/«`	t%jx4X@:2Ymi
/vHq&@E.WwЋ1,KT={rSr,
_+^#c|e]1.0^g86x/@ ! ^x/ ^x/B1FƘv4L9_5pR=Fc-}g[|_Vb+K'X8ХgYǨ8VId5وHiPK1qNFy+7RbT*3Ò4m^c,H5huWse钉a49\ڳvGb]I.]95B#.pKmtzn¤	?׸	ar	(WSzy^ȲhK#F;meQ\Ky`)n&˲>i+)ť1a>	Oz9'8'Z]F	kыkQbK
*ȵeȼ^8[e+aln0'GX-jr-%8A U,g%t[MRd?:_ڬ(/׫c)i{KR|A-oKIvK.:*lت!n%:˛`k,KG@.X0VCi.%nHK	I.EŸZ!ĹQNw*+[⁃]i|lJmy"tk0^m0(U^,'Z
k=.Wab-%iBKI[0VYɹ.%$|*A^v{TUā.%rP 4g\vzoc{Zcb,K9kT].%OX`{|]i-wOjmPEY(i1:QuQ_ATd6sz]%(V)[	.E7szx*mڪu1k:[m{ګr[2ΈF@)V95
m))%[Ktjx%cGT]cWRDd)*	yJ}¹RjK1q" :S5>ܖ>̭Kn7@	,R]QqK)<ХK^TVjIJoKqb,'Cja]X'4GD: VeܶӶ'RP"3<[0*)tlhK)}**ηUZd	N)CBeP^ɩÍ٪-nh
Q:j@r&s)uKoyB[妹w@3DJKvwkk|NXk۾۴LަݡnE:Z艁7ki(RR'(ului;\Ó8ߨ	%Y.)e~%ti܄0.93ْ+RlK.}U}i1/^mfYjl[r<8Q=FFzՕb^`cԯ5߄ةߵ,K.4p}wlXs1N`6vKGŁ3@nz"r[	.%$:gdۊR!6ޥΓj	g*-ԔT]f)yP!F]C*l;7<Rx4N9q[Q}5=T;e6cjk<#)EDX%
Js)iKy9ya:b5cT^jQ+|#:ۮnߵݦ^Om:?nBO
SLS14í^US UFyvgMK[*߃ka%]cc-%:A?k㫑812.pkNK5BÒ=QI/`#*-6x2s'()4*+}4)<#,˒rNzU^j,.y
/z [V٦bg\sUV)VVVwyQ~ߢb,
VD?f]J(>iPP41ﷶ;GN
c^yrkWUd~ޠ*?ƖkPtFjY6@R\w1(fjYѮ=l@CxkpYz#вxUpVXӦF2^bY<Pjh1*.7UU7%6BSqM#7j%ZZ{xd+6;; ^24x/@^ ^x@ ^x@@(AcIENDB`
PNG


IHDRiCCPICC profile(}=H@_SZv(␡:Yq*BZu04$).kŪ "%/)=B4ktL%b&*^!A""3˘$:{zY9ԜH<& ޴
aVUs1.Hu76өy0XhcYԈSXY+WYuH`K BA%a#FN;\D.\%0r,
Z	/)_c|;N?Wz_3ZZ.[\'C6eWy)5>N4uewxwO{oiry]bKGD	pHYs+tIME
DwIDATxypU׹7}PDJ[HvJm8b]F\:TFEjԱ-uCb_h"` ,!='%jM|d|g
|>1\x/@ x/@ x/@ x/@ x/@ x/R8E7t6l4?ϟqƵk]{rg}w}W3fh׺;M6?Tnn+uپk2dF뮻N3fP;Ǐk̘1?,LVZZF9sh֬Y}뮻O|ꫯBxYkȐ!6m͛;Trrr4h 3C0@/i$QF^zx69s7T^voٲMmۼfر!q>7lrrr}z5insĉ.ceY檫2;vнԓ-_yyyGBI	_J]vviO>DW\q-Z?<$Zׯѣן:mذAcƌ}ݧ.NEڹs&O믿^_~%a^8p@?n:F^|EM<Y~iHn_MMVX)S(??OzW_+6mڤ'j׮]



;t]wq0Xln喐S>'}|駺u!. jԩzW9;{9ek֬SO=գ9''GsQaaa9OgΜܹsޤ^/@߱l2m޼q۷{~I-XO~}xPWW 0,zS*<<1F:ydP5m[oUfff
dɒS\\F$\.ۦ<=z4ڵk}Y76L^xaǣ<eff;wԟ'qv+m[Reee{sGEDDBU'KVVV۸w^3}YbE%_]Òάkرf۶m}*..6W6/>>u]CIIyLLLL}NNNxud^zi:oQFuڵCÒ>Kq'N֭[n:EEEmjժznƌ
^vv>~۸n=4}t?B%&&jҥڿjmOk8!!Awy<)Sm[TTիW{>""Bsվ}twȲ,~a~Q!teY>]YYw}֬YTVVqv>|iŊZre94##C۷oׄ	{Wda+.Tg?<ߢf{~ر#>^6g-^o^{?KNNjܹ}ܾ}nFٶC@RR7~[o}v0ˊlMNNkaۭ_~YkѣGCfrPJ7xxcՊmu-[#3K/
]viѢEzݲo4F'x5qo^xAn;d_0oٲW]cƌ
O9^;4{VWWWcdXm޼Y׿i&z}4{jViucǎرcC.Z^wǏsc<x0 }.?u7~m۶iРA׿~z=Cm4clxbZ[;\tE~S3o)Ƿ]}eY7nE ^Α#Gj~K"\R<LKyyf͚'Ol;w\.SRRBwzu<p6_߮V$jP^vez뭷5{+t~i޼yڿW^y	
[(eWo@3eo81F7tmz\;v/;,r/'?њ5kx<Zp۩7nСںu8i/#lWUUkV˗/ן풒uVs^裏5YQQfΜ֬Yp_əe;>^AYn.\]vvfΜ)7nܨKl^{-xw\k&`?\sUMMMޣ-SK5k'УqsY3Add6mڤɓ'k߾}~6MAo5g}֦)_pȑ#CrV\믿/>}>-[?N8o%Kh~;vL^{mPS?Nk/m+//GD|iii^A/|V9sF3gz+Wrr^裏gϞVViīV);s`@qY-^X1jh6lhVӁ6) }@CCNeeem7o޼N{϶LAxbmݺU^W7pSw!M4I/v={vw[ ^p!S$VĉAձv6c***t	{ڰanŊ
۔{ojmڴ)2:thHn[LL'^}{1:i$g???ߡ0e0Z."@hƍ,KgڵAMAܚpL h[|˟$25M<sLN 8ڼyMS_zmzSdڽ{Nz߻i
⌌3e0Masʕ:p.nێ` f`@P&N'xBYYYzԯ_nߦ#GjѢE~%Ct-5nWSZZ4a};	chd9'3yސm[gL>1@=S@~vcn[sW/ۭ$	^3߿?$x<.OKKu*33쁽q;.OMM ,;]tW_}P-g}6l9>Cr/HSL[nm!G~;mM֫ѣGOm3c.Fuuu9^_~V׫nIIJJҏ~^s~sG-_<_&]zӧ|vSNiڻwv,/ԬY{n~ӟ}}0f? L:mg߿-[MCCq]~ھ}yT=.622R=P^˵n:7N׿ׯ_w3yVX/% D0,#ayn]C>3jôiѣG+11
x<Sffz͝wީÇw}|M}󕙙toҥK5dȐnF?ٳgw{۶b=zTAfҤIZ`Aڰa
/gZf.\wLoG;legg6vX=ݺ
ڲeK8^z>]}}=R(ip^͟?_=X8;}jғDmڴI\A,K_5vX@З-_\7|shy[o.`Cu?A`^^|E==b#FO>ԩS9rvޭ5QQQz7tR@e˖i!krt7jϞ=}O.K7|ӧz;kݻ5o<xh
u?eY
ok<
A?3t!_^Vɓ'n͚5K+V3㨆i̙z't%p.#<B讟WtzD[mkZuu޵caK,ђ%Ko>裏\tiKMMUZZFٳgkΜ9JJJiٳgSbb"7h+&t]s5Z`@O9_?꿵/«`	t%jx4X@:2Ymi
/vHq&@E.WwЋ1,KT={rSr,
_+^#c|e]1.0^g86x/@ ! ^x/ ^x/B1FƘv4L9_5pR=Fc-}g[|_Vb+K'X8ХgYǨ8VId5وHiPK1qNFy+7RbT*3Ò4m^c,H5huWse钉a49\ڳvGb]I.]95B#.pKmtzn¤	?׸	ar	(WSzy^ȲhK#F;meQ\Ky`)n&˲>i+)ť1a>	Oz9'8'Z]F	kыkQbK
*ȵeȼ^8[e+aln0'GX-jr-%8A U,g%t[MRd?:_ڬ(/׫c)i{KR|A-oKIvK.:*lت!n%:˛`k,KG@.X0VCi.%nHK	I.EŸZ!ĹQNw*+[⁃]i|lJmy"tk0^m0(U^,'Z
k=.Wab-%iBKI[0VYɹ.%$|*A^v{TUā.%rP 4g\vzoc{Zcb,K9kT].%OX`{|]i-wOjmPEY(i1:QuQ_ATd6sz]%(V)[	.E7szx*mڪu1k:[m{ګr[2ΈF@)V95
m))%[Ktjx%cGT]cWRDd)*	yJ}¹RjK1q" :S5>ܖ>̭Kn7@	,R]QqK)<ХK^TVjIJoKqb,'Cja]X'4GD: VeܶӶ'RP"3<[0*)tlhK)}**ηUZd	N)CBeP^ɩÍ٪-nh
Q:j@r&s)uKoyB[妹w@3DJKvwkk|NXk۾۴LަݡnE:Z艁7ki(RR'(ului;\Ó8ߨ	%Y.)e~%ti܄0.93ْ+RlK.}U}i1/^mfYjl[r<8Q=FFzՕb^`cԯ5߄ةߵ,K.4p}wlXs1N`6vKGŁ3@nz"r[	.%$:gdۊR!6ޥΓj	g*-ԔT]f)yP!F]C*l;7<Rx4N9q[Q}5=T;e6cjk<#)EDX%
Js)iKy9ya:b5cT^jQ+|#:ۮnߵݦ^Om:?nBO
SLS14í^US UFyvgMK[*߃ka%]cc-%:A?k㫑812.pkNK5BÒ=QI/`#*-6x2s'()4*+}4)<#,˒rNzU^j,.y
/z [V٦bg\sUV)VVVwyQ~ߢb,
VD?f]J(>iPP41ﷶ;GN
c^yrkWUd~ޠ*?ƖkPtFjY6@R\w1(fjYѮ=l@CxkpYz#вxUpVXӦF2^bY<Pjh1*.7UU7%6BSqM#7j%ZZ{xd+6;; ^24x/@^ ^x@ ^x@@(AcIENDB`