`ccache` GitHub action generates excessive caches

[encukou]

The ccache action speeds up *nix builds considerably (though not enough that we could do without it).
The issue with it is that every build uploads a new cache, filling up our quota very quickly. GitHub removes caches that weren't used recently, but ccache is aggressive enough that it's crowding out smaller, useful entries. Specifically, the action to mark stale-PRs (which only runs every once in a while), has its state removed before the next run.

• Solve this in main. (It's hard to test locally, iterating on main seems to be the best way to do it. Sorry for the noise!)
• Backport to bugfix branches

Linked PRs

• GH-113858: GitHub Actions config: Only save ccache on pushes #113859
• gh-113858: Cut down ccache size #113945
• [3.12] gh-113858: GH Actions: Make ccache smaller (GH-113859, GH-113945) #114082
• gh-113858: GH Actions: Limit max ccache size for the asan build #114113
• [3.11] gh-113858: GH Actions: Make ccache smaller (GH-114082) #114188

[encukou]

It's not possible to change the contents of an existing cache, so for this to be useful we need to save a new one occasionally.
With #113859 it should save only on merged commits, not in-progress PRs. Hopefully that'll be enough.

[hugovk]

Another idea is to set a max-size, but the above PR looks like a good thing to try first 👍

https://github.com/hendrikmuhs/ccache-action/blob/4cb35b2ff44ce57e1f5f5221e26bd8fefcfe41d0/action.yml#L15-L17

[encukou]

Some napkin math suggests that this should be enough, at least outside sprints. Which is good enough :)
I don't think we want to delete the caches manually, so we'll be at GitHub's 10GiB limit regardless of max-size.

I plan to merge tomorrow morning (EU time) so I have the whole day to watch the effect (and revert if needed).

[erlend-aasland]

Is there more to do here? (I accidentally clicked "Close with comment" iso. "Comment"; so, please reopen if there is more to do here 😄)