Improve CI builds using matrix strategy

[robert-scheck]

• Consolidate separate clang and gcc actions into one using matrix strategy
  • Use latest clang and gcc
  • Use the last but one, too
• Switch from deprecated actions/checkout@v2 to actions/checkout@v3
• Add cronjob-like build to catch possible build failures with new package versions
• Allow to run a workflow manually using workflow_dispatch option

[adulau]

That's a nice improvement for the CI. Thank you very much for the contribution!