Migrate from CircleCI to GitHub Actions

[Max Jonas Werner]

One less external system to care about.

[Ciaran Moran]

We'd like to retire our CircleCI subscription this year.

[Ciaran Moran]

There's no huge rush, as we have 6.8m pre-paid credits to use, and this is using ~6 / month. But it will fail when we terminate CircleCI.

[Max Jonas Werner]

There's a PR up now to get rid of CircleCI: #54