flutter bot is getting to be painfully slow

[jcollins-g]

It's been the long pole in dartdoc's CI for a while, but it is much worse now than it used to be. Investigate why, because dartdoc performance itself doesn't seem to be to blame...

[jcollins-g]

Well, dartdoc performance regressions aren't likely to blame, but it may be that the transition to Github Actions means the bots are running on lower-performing VMs.

Looked at a sample run, and initialization took 4m (mostly building Flutter from scratch), 35 minutes to build Flutter documentation, and 1.5 minutes to build the Flutter test package.

More aggressive caching could deal with the initialization time, but the bulk is still dartdoc.