Revert "python: skip build at install phase"

This reverts commit d34f5b657051e268e41084e9751996d16f7f0a1a.

PIL and NumPy don't like this change and I'd rather spend time on
bringing wheels to Nix than fixing old infrastructure.

pkgs/development/python-modules/generic/default.nix

@@ -118,7 +118,6 @@ python.stdenv.mkDerivation (builtins.removeAttrs attrs ["disabled"] // {
     ${python}/bin/${python.executable} setup.py install \
       --install-lib=$out/lib/${python.libPrefix}/site-packages \
       --old-and-unmanageable \
-      --skip-build \
       --prefix="$out" ${lib.concatStringsSep " " setupPyInstallFlags}
 
     # --install-lib: