CI rework about metatype

[Yohe-Am]

@zifeo what do you think of merge queues or alternatively Bors?

Also, what about only running the full CI when user wants to merge the PR instead of every push?

[zifeo]

@Yohe-Am

sccache: we definitely must try, however I am not 100% sure it will work well because of those caveats, yet let's give it a try. Happy to set up a R2 backend when you have some time to move forward with that one.

merge-queue: we have played a bit with them and we can re-use them (should already be available) now that we have shorter pipeline. Bors could be an alternative but I am not sure to see the advantage with GitHub ones?

[Yohe-Am]

@zifeo Hey, so I'm taking another look at merge queues. We had another PR that broke main after merge which would have been caught by the feature. It should also help when there are a lot of PRs in flight (I expect the median PR size to get smaller now that we're doing a lot more polish). Should we give them a try at the end of this cycle?