CI: Switch to macos-12 runner
Github now has a macos-12 runner: https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners
We should switch our macOS run to use it.
Github now has a macos-12 runner: https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners
We should switch our macOS run to use it.