[!80] [CI] Refactor pacxx-build jobs

Merge branch 'ci-refactor-pacxx-build' into 'master'

Addresses: [#32]

See merge request [joe/pacxx-docker!80]

  [#32]: gitlab.dune-project.org/joe/pacxx-docker/issues/32
  [joe/pacxx-docker!80]: gitlab.dune-project.org/joe/pacxx-docker/merge_requests/80