feat: add CI3 flake detection comments to PRs

[ludamad]

automatically post/update a comment on pull requests listing all flaked tests

[spalladino]

IIRC there was a gh action for managing sticky comments? Still, if we want to handle everything from sh, this makes more sense.

[ludamad]

I really think this is the right way of doing things. I was able to test all the pieces working, our mainframe env is pretty much identical to our CI env. IOU: something better than shell. For now, I'm experimenting with new stuff in shell-y Python. (Basically anything we use will be calling out to other programs a lot, though, so I'm not giving huge promises here about being 100% idiomatic in whatever we may decide to start using) Practically speaking, with our current setup it's not worth communicating between the runner and github actions.