Make CI config excellent

[szepeviktor]

• do things in chronological order
• be explicit
• be faster

[shochdoerfer]

I had on my todo list to rework the whole GitHub Actions setup. I won't merge your changes because when I do, I will update our other projects as well. I need consistency among our project setups and don't want to deal with custom changes just because someone has a different preference for certain things.

I do appreciate some of your findings, so thanks for that.