Revert "Revert "Revert "cpython: fix finding headers when cross-compiling extension modules"""

This still does not function without issues. E.g., bootstrapped-pip fails with Python 3.6 and 3.7
as well as 3.8 on 32-bit.

Because this is a stdenv-rebuild it needs to be tested significantly better

This reverts commit 6100bc29f7.

pkgs/development/python-modules/bootstrapped-pip/default.nix

@@ -45,7 +45,7 @@ stdenv.mkDerivation rec {
     mv wheel* wheel
     # Set up PYTHONPATH. The above folders need to be on PYTHONPATH
     # $out is where we are installing to and takes precedence
-    export PYTHONPATH="$out/${python.sitePackages}:$(pwd)/pip/src:$(pwd)/setuptools:$(pwd)/setuptools/pkg_resources:$(pwd)/wheel:$PYTHONPATH"
+    export PYTHONPATH="$out/${python.sitePackages}:$(pwd)/pip/src:$(pwd)/setuptools:$(pwd)/setuptools/pkg_resources:$(pwd)/wheel"
 
     echo "Building setuptools wheel..."
     pushd setuptools