Add separate cache for each branch in CI

That should solve the issue you described to me today. It may lead to longer build times for the next couple of builds, because no properly named caches are found and need to be rebuilt.

There might be still an issue when merges to the same branch happen shortly after each other. If we want to keep all individual builds for that branch, there seems to be a way now to prevent race conditions by creating a resource manage that from the CI configuration, essentially working as a lock. I have not tried that yet, but it would be an option.