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.
# -*- Makefile -*-
####################################################################
#  LAPACK make include file.                                       #
#  LAPACK, Version 3.2.1                                           #
#  June 2009		                                               #
####################################################################
#
# See the INSTALL/ directory for more examples.
#
SHELL = /bin/sh
#
#  The machine (platform) identifier to append to the library names
#
# WA for WebAssembly
PLAT = _WA
#
#  Modify the FORTRAN and OPTS definitions to refer to the
#  compiler and desired compiler options for your machine.  NOOPT
#  refers to the compiler options desired when NO OPTIMIZATION is
#  selected.  Define LOADER and LOADOPTS to refer to the loader
#  and desired load options for your machine.
#
#######################################################
# This is used to compile C library
#CC        = gcc  # inherit $CC from emmake
# if no wrapping of the blas library is needed, uncomment next line
#CC        = gcc -DNO_BLAS_WRAP
CFLAGS    = -O3 -I$(TOPDIR)/INCLUDE -fPIC -DNO_BLAS_WRAP
LDFLAGS	  = -O3
LOADER    = $(CC)
LOADOPTS  =
NOOPT     = -O0 -I$(TOPDIR)/INCLUDE -fPIC
DRVCFLAGS = $(CFLAGS)
F2CCFLAGS = $(CFLAGS)
#######################################################################

#
# Timer for the SECOND and DSECND routines
#
# Default : SECOND and DSECND will use a call to the EXTERNAL FUNCTION ETIME
# TIMER    = EXT_ETIME
# For RS6K : SECOND and DSECND will use a call to the EXTERNAL FUNCTION ETIME_
# TIMER    = EXT_ETIME_
# For gfortran compiler: SECOND and DSECND will use a call to the INTERNAL FUNCTION ETIME
# TIMER    = INT_ETIME
# If your Fortran compiler does not provide etime (like Nag Fortran Compiler, etc...)
# SECOND and DSECND will use a call to the Fortran standard INTERNAL FUNCTION CPU_TIME
TIMER    = INT_CPU_TIME
# If neither of this works...you can use the NONE value... In that case, SECOND and DSECND will always return 0
# TIMER     = NONE
#
#  The archiver and the flag(s) to use when building archive (library)
#  If you system has no ranlib, set RANLIB = echo.
#
ARCH     = $(AR)
ARCHFLAGS= cr
#RANLIB   = ranlib
#
#  The location of BLAS library for linking the testing programs.
#  The target's machine-specific, optimized BLAS library should be
#  used whenever possible.
#
BLASLIB      = ../../blas$(PLAT).a
#
#  Location of the extended-precision BLAS (XBLAS) Fortran library
#  used for building and testing extended-precision routines.  The
#  relevant routines will be compiled and XBLAS will be linked only if
#  USEXBLAS is defined.
#
# USEXBLAS    = Yes
XBLASLIB     =
# XBLASLIB    = -lxblas
#
#  Names of generated libraries.
#
LAPACKLIB    = lapack$(PLAT).a
F2CLIB       = ../../F2CLIBS/libf2c.a
TMGLIB       = tmglib$(PLAT).a
EIGSRCLIB    = eigsrc$(PLAT).a
LINSRCLIB    = linsrc$(PLAT).a

# -*- Makefile -*-
####################################################################
#  LAPACK make include file.                                       #
#  LAPACK, Version 3.2.1                                           #
#  June 2009		                                               #
####################################################################
#
# See the INSTALL/ directory for more examples.
#
SHELL = /bin/sh
#
#  The machine (platform) identifier to append to the library names
#
# WA for WebAssembly
PLAT = _WA
#
#  Modify the FORTRAN and OPTS definitions to refer to the
#  compiler and desired compiler options for your machine.  NOOPT
#  refers to the compiler options desired when NO OPTIMIZATION is
#  selected.  Define LOADER and LOADOPTS to refer to the loader
#  and desired load options for your machine.
#
#######################################################
# This is used to compile C library
#CC        = gcc  # inherit $CC from emmake
# if no wrapping of the blas library is needed, uncomment next line
#CC        = gcc -DNO_BLAS_WRAP
CFLAGS    = -O3 -I$(TOPDIR)/INCLUDE -fPIC -DNO_BLAS_WRAP
LDFLAGS	  = -O3
LOADER    = $(CC)
LOADOPTS  =
NOOPT     = -O0 -I$(TOPDIR)/INCLUDE -fPIC
DRVCFLAGS = $(CFLAGS)
F2CCFLAGS = $(CFLAGS)
#######################################################################

#
# Timer for the SECOND and DSECND routines
#
# Default : SECOND and DSECND will use a call to the EXTERNAL FUNCTION ETIME
# TIMER    = EXT_ETIME
# For RS6K : SECOND and DSECND will use a call to the EXTERNAL FUNCTION ETIME_
# TIMER    = EXT_ETIME_
# For gfortran compiler: SECOND and DSECND will use a call to the INTERNAL FUNCTION ETIME
# TIMER    = INT_ETIME
# If your Fortran compiler does not provide etime (like Nag Fortran Compiler, etc...)
# SECOND and DSECND will use a call to the Fortran standard INTERNAL FUNCTION CPU_TIME
TIMER    = INT_CPU_TIME
# If neither of this works...you can use the NONE value... In that case, SECOND and DSECND will always return 0
# TIMER     = NONE
#
#  The archiver and the flag(s) to use when building archive (library)
#  If you system has no ranlib, set RANLIB = echo.
#
ARCH     = $(AR)
ARCHFLAGS= cr
#RANLIB   = ranlib
#
#  The location of BLAS library for linking the testing programs.
#  The target's machine-specific, optimized BLAS library should be
#  used whenever possible.
#
BLASLIB      = ../../blas$(PLAT).a
#
#  Location of the extended-precision BLAS (XBLAS) Fortran library
#  used for building and testing extended-precision routines.  The
#  relevant routines will be compiled and XBLAS will be linked only if
#  USEXBLAS is defined.
#
# USEXBLAS    = Yes
XBLASLIB     =
# XBLASLIB    = -lxblas
#
#  Names of generated libraries.
#
LAPACKLIB    = lapack$(PLAT).a
F2CLIB       = ../../F2CLIBS/libf2c.a
TMGLIB       = tmglib$(PLAT).a
EIGSRCLIB    = eigsrc$(PLAT).a
LINSRCLIB    = linsrc$(PLAT).a
