This makes our dependent tests run on a GitHub macOS runner if
compatible with the requested timeout.
This will also allow builds on a GitHub runner on macOS if requested
with the environment variable `HOMEBREW_MACOS_BUILD_ON_GITHUB_RUNNER`.
This environment variable will need to be set in the workflow as
appropriate (e.g. when requested by a label).
This will allow us to set a slightly longer timeout on Intel Big Sur,
which is useful because the Intel Big Sur runner often exceeds our short
time limit by itself.
The `GitHubRunnerMatrix` takes a list of testing formulae, deleted
formulae, and the available runners and constructs the matrix of active
runners for any given test job.
