Enable MacOS tasks for high-priority backlog tasks (#1805)

Due to the outage we went without MacOS coverage for three days, which
means that there is a significant backlog. This change enables MacOS
jobs for high-priority jobs in order to help us clear the backlog.
Hopefully we can enable MacOS for all jobs soon.

https://github.com/bazelbuild/continuous-integration/issues/1800
1 file changed