[CI] Use pacxx-docker's getpacxx
See https://gitlab.dune-project.org/joe/pacxx-docker/issues/20
WIP: Can't work before https://gitlab.dune-project.org/joe/pacxx-docker/merge_requests/60 has hit master
See https://gitlab.dune-project.org/joe/pacxx-docker/issues/20
WIP: Can't work before https://gitlab.dune-project.org/joe/pacxx-docker/merge_requests/60 has hit master