CI fixes

[pmbittner]

• make CI interruptible: So when we push, and there is a CI job running, then interrupt the current CI job and restart it instead of crashing.
• enable manual workflow runs on Github, see:
  • https://docs.github.com/en/actions/managing-workflow-runs-and-deployments/managing-workflow-runs/manually-running-a-workflow
  • https://docs.github.com/en/actions/writing-workflows/choosing-when-your-workflow-runs/events-that-trigger-workflows#workflow_dispatch

[pmbittner]

Thank you! :)