Use common script to get pacxx for CI

I'm sick and tired, sick and tired, of repeating myself in every ci script.

Note: the difference to getpacxx from https://zivgitlab.uni-muenster.de/HPC2SE-Project/pacxx is, that that script is meant more for documentation and teachability, while I'm going to need some more features. For instance the ability to substitute the repository in the current directory for one of the to-be-fetched repositories, because the CI system already fetched it. Including that feature into the https://zivgitlab.uni-muenster.de/HPC2SE-Project/pacxx script would reduce that scripts simplicity.

• write script (!60 (merged))
• use in project's CI (that has a working CI yet)
  • https://gitlab.dune-project.org/joe/pacxx-docker (!60 (merged))
  • https://zivgitlab.uni-muenster.de/HPC2SE-Project/pacxx (does not make sense here, really that repositories scripts should be tested)
  • https://zivgitlab.uni-muenster.de/HPC2SE-Project/pacxx-runtime (pacxx-runtime!32)
  • https://zivgitlab.uni-muenster.de/HPC2SE-Project/pacxx-samples (N/A, uses the compiled pacxx from the docker images)
  • https://zivgitlab.uni-muenster.de/HPC2SE-Project/pacxx-llvm2 (pacxx-llvm2!5)
  • https://zivgitlab.uni-muenster.de/HPC2SE-Project/pacxx-rv2 (pacxx-rv2!2)
  • https://zivgitlab.uni-muenster.de/HPC2SE-Project/pacxx-clang (pacxx-clang!2)