Re-enable CI cache

[Kobzol]

Proposed in https://rust-lang.zulipchat.com/#narrow/channel/185405-t-compiler.2Frust-analyzer/topic/Moving.20off.20merge.20queue.20to.20new.20bors.3F/with/598734807.

It should now actually work, because CI was switched to run also on the master branch. The coverage workflow seemed to be quite slow (~5 minutes), so I also enabled the rust-cache action for it; the workflow was already running on pushes to master.

I removed two commented (previous) usages of rust-cache, because the corresponding jobs seem to be quite fast these days (<1m).

I pinned the version to Swatinem/rust-cache@c193711, which is the latest released version of rust-cache as of today, 2.9.1.

[ChayimFriedman2]

What is the expected speed increase? If it doesn't turn CI into "immediate", and there aren't other reasons to have a cache (save compute for the Project etc.), I don't think this is worth it. Having to deal with caches in CI can be not fun, and 10 minutes is not a lot (r-l/r CI is some hours - that is too long).

[Kobzol]

Lukas wrote that CI times are atrocious, so I suppose that it depends on the perceived speed 😆 My goal would be to get all workflows, say, under 2 minutes, so that there isn't any large bottleneck standing out. From my experience, the difference in CI time between 2 minutes and 6-8 minutes is quite large. Not sure if we can get down to 2 minutes though.

[ChayimFriedman2]

Maybe it depends on the workflow. I do not depend on CI usually, I run on checks locally (test+Clippy, of course not on all platforms but it's usually enough), unless I have a strong reason to believe I'm doing something simple that will succeed), then publish the PR and turn around to do something else. After some time I check back on the PR.