Move CI/CD workflows to GitHub Actions

[callahad]

@arbourd Please work with folks in SysEng to define milestones and work to be done to get CI/CD up and running on GitHub.

The ultimate goal is to end use of the unit-try repo (or at least the Mercurial incarnation of it), but I'd love to see a phased / incremental approach that gives us quick wins sooner.

One thing you'll need to tackle: Do we decommission BuildBot entirely, or do we use webhooks to trigger BuiltBot and report back via the GitHub Checks API?