Ci improve caches

[Profpatsch]

better action caching for the cargo build & ci check

Checklist

• [ ] Updated the documentation (code documentation, command help, ...)
• [ ] Tested the change (unit or integration tests)
• [ ] Amended the changelog in release.nix (see release.nix for instructions)

[Profpatsch]

I can’t get the carnix cache working, so I guess we’ll just keep downloading all deps for now :facepalm: