Disable MacOS on PRs

[jaredpar]

Our PRs already run tests on Unix which is a sufficient proxy for Mac runs. Can leave Mac as CI only and slim down our PR runs

[RikkiGibson]

It was at least true at one time that Mac runs queued much faster than Linux. Do we know the state of that now?

[jaredpar]

It was at least true at one time that Mac runs queued much faster than Linux. Do we know the state of that now?

It runs faster but has far less capacity. This means when you measure CI / PR time switching from Unix to Mac you almost always see a win. But when you actually commit it a few days later you will see a loss. That cause a single PR doesn't stress the capacity issue but the weight of our dev team does. It's a lesson several of the repos have learned at this point.

The other issues with Mac:

• The queue itself has more frequent outages than Unix
• Not as many teams depend on it hence it has lower priority when things are broken. Contrarily when Unix is down the .NET world is down so no need for us to push on a "please fix" cause literally everyone else is.