fix skbuild metadata installation path for external modules

I tried to deploy a pypi package, which is build with scikit-build. This did work out since it was unable to find some headers. I derived that this fixes the issue with packages registered via registerExternalModule.

Now during pip install scikit-build has the correct CMAKE_FLAGS available. This could also be an issue for dumux but there is no pypi package, is it? Therefore this issue never arose? @tkoch
@andreas.dedner Can you also have a look? Thanks! Is this something that should be backported?