MAINT improve isolation for micropip tests. I added a new `wheel_base` fixture which creates a new temporary directory and mocks `site.getsitepackages()` to return it. I also removed the `WHEEL_BASE` global variable from micropip and instead calculate it each time in `micropip.install`.
package:
  name: libiconv
  version: "1.16"

source:
  url: https://ftp.gnu.org/pub/gnu/libiconv/libiconv-1.16.tar.gz
  sha256: e6a1b1b589654277ee790cce3734f07876ac4ccfaecbee8afa0b649cf529cc04

build:
  library: true
  script: |
    emconfigure ./configure \
       CFLAGS="-fPIC" \
       --disable-dependency-tracking \
       --disable-shared \
       --prefix=${WASM_LIBRARY_DIR}

    emmake make -j ${PYODIDE_JOBS:-3}
    emmake make install

package:
  name: libiconv
  version: "1.16"

source:
  url: https://ftp.gnu.org/pub/gnu/libiconv/libiconv-1.16.tar.gz
  sha256: e6a1b1b589654277ee790cce3734f07876ac4ccfaecbee8afa0b649cf529cc04

build:
  library: true
  script: |
    emconfigure ./configure \
       CFLAGS="-fPIC" \
       --disable-dependency-tracking \
       --disable-shared \
       --prefix=${WASM_LIBRARY_DIR}

    emmake make -j ${PYODIDE_JOBS:-3}
    emmake make install
