Limit regression test workers to 1

[srittau]

This should address #11220.

[srittau]

It's fairly slow now. I wonder whether we could reduce the amount of tests run. For example, run only on Python 3.8 and 3.12. In the long run it could also make sense to separate out tests that need to run on multiple platforms or Python versions as currently we are running lots of redundant tests.

[AlexWaygood]

And/or we can shard it.

I'm also experimenting locally with whether adding the --no-incremental flag fixes things, since the race condition (if it is a race condition) is likely due to different mypys concurrently trying to read the same cache. --no-incremental will probably make running with --all a fair bit slower locally, but probably won't make much difference in CI, so it's probably a pretty good tradeoff (I doubt many people run with --all much locally anyway; I certainly don't).