But it's not an _error_ --- github just waits forever for a runner
to be available? Anyway, from
https://github.blog/changelog/2022-04-25-github-actions-public-beta-of-macos-12-for-github-hosted-runners-is-now-available/
it looks like `macos-12` should work.
There's also `macos-latest`, but it sounds like that's still
`macos-11` atm, so although we should probably come back and switch
to that, it's a bit early yet.
Although we could switch to `macos-latest` but explicitly say that
we need the homebrew coreutils installed?
Needed because the build now relies on `readlink -f`, but I don't
think we really care about old versions of macOS anyway, since Mac
users upgrade pretty quickly? It's a bit odd that the _current_
version wasn't being tested in CI, but macOS 10.15 did still make
some sense because it's the oldest version that's still getting
security updates.
Update the documentation and github CI actions (seeing all failures on
github makes sense because most people looking at failing Darwin test
runs there won't be able to reproduce locally for lack of a Mac).
