[CI] Use pacxx-docker's getpacxx
Stolen from pacxx-llvm2's .gitlab-ci.yml with minimal modifications.
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
Edited by Dr. Jorrit Fahlke