Try to cache the clean version of flutter Dartdoc uses

[jcollins-g]

Perhaps fixes the infrastructure part of #2799.

This could speed up most runs for the flutter bot by a few minutes. The cache is very conservative, being sure to throw away once a day just in case of accumulated cruft.

Uncached run: https://github.com/dart-lang/dartdoc/pull/2802/checks?check_run_id=3665189847

Cached run: https://github.com/dart-lang/dartdoc/pull/2802/checks?check_run_id=3665918707

[jcollins-g]

Definitely improved the runtime, so mailing out.

[jcollins-g]

I will go ahead and land this as speeding up these bots has immediate, noticeable benefits. But @athomas if you have some comments or suggestions on how to improve this, I'll implement in a follow up.

[athomas]

LGTM, perhaps the .pub-cache cache could also be used on other jobs than flutter (not sure if it should be shared between jobs, probably not across OSs at least so the key would need to include the OS).

You may also want to upgrade setup-dart to 1.2 (there's some fix with git dependencies on Windows in that version).

What happened to the coverage, did that stop working?

[jcollins-g]

@athomas WRT .pub-cache, yeah I know I don't want the flutter cache sharing the others as Flutter needs far more pub packages than the main test. The others using a Flutter-test generated cache though would be fine. Not sure how to make GitHub Actions do that.

I'll look into updating setup-dart. I disabled coverage because it kept breaking and I wasn't getting enough benefit from coverage for it to be worth the time I was investing in the infrastructure.