Test a custom PyPI implementation

This uses a local drop-in replacement for PyPI. It allows us to run our CI on any runner that has unrestricted access to gitlab.dune-project.org - not just those that whitelist PyPI access.

The replacement index is implemented here: https://gitlab.dune-project.org/dominic/package-index/-/tree/master

Currently, the additional index is hardcoded into https://gitlab.dune-project.org/quality/dune-testtools/-/blob/feature/custom-pypi-ci/CMakeLists.txt#L26, but as soon as core/dune-common!919 (merged) lands, this can be moved into CI configuration.