Revert "python: skip build at install phase"

This reverts commit d34f5b6570.

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: