buildPythonPackage: install builds reproducibly

Without --build a temporary directory is created to unpack the wheel in.
We now set a directory instead.

pkgs/development/interpreters/python/build-python-package-common.nix

···
     export PYTHONPATH="$out/${python.sitePackages}:$PYTHONPATH"
 
     pushd dist
-    ${bootstrapped-pip}/bin/pip install *.whl --no-index --prefix=$out --no-cache ${toString installFlags}
+    ${bootstrapped-pip}/bin/pip install *.whl --no-index --prefix=$out --no-cache ${toString installFlags} --build tmpbuild
     popd
 
     runHook postInstall