Implement workaround to omit the `--system-site-packages` flag of the virtualenv

Having system site packages in the env screws us in many ways, especially if users have the idea of installing dune python packages globally!

The reason to use system-site-packages in the first place is to allow to mix globally installed dune-python packages with ones that are installed into the virtualenv.

One possible solution idea that came up in the past:

During "make install" , we could write an installation script that allows to pip install the dune python packages.