Factor out common CI stuff

[smheidrich]

Three options for how:

• Split up into three workflows (build, pre-release, release), make the two release ones get artifacts from the build one
  • drawback: passing artifacts between workflows is not supported by GitHub, needs possibly flaky 3rd party tools like https://github.com/dawidd6/action-download-artifact/
• Put common stuff into an action of its own
  • drawback: a lot of work for not much gain
• Merge everything together into one workflow again (triggers on both branches: [ main ] and tags: [ "[0-9].*" ]), just have a couple of conditions for differentiating between release / pre-release
  • drawback: boring, might not work, I don't know