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.
We don't want to define `MacOS` in the generic OS, so let's make sure we
refer to it as `OS::Mac` in the places we need it.
This reverts commit 575cb0263cb1428c63b9428bbd913ffde83bd317.