Let Each macOS Arch In on Its Own Without Holding an Idle Runner

Letting a pull request run in whole deadlocks when its second arch
reaches the line behind another pull request: the run holds two
runners, the pull request in front waits for them, and the second arch
never gets to check. Each arch now waits on its own. A run with one
arch let in holds a runner only until that arch's app build is done,
and a run with both holds two until its universal build and tests
finish, so whoever is first in line always gets in eventually.

Co-authored-by: raistlin7447 <kris.austin@gmail.com>

.github/workflows/build_check_cache.yml

@@ -143,6 +143,7 @@ jobs:
     env:
       WAIT_MINUTES: 240
       PRIORITY: ${{ contains(github.event.pull_request.labels.*.name, 'macos-priority') }}
+      ARCH: ${{ inputs.arch }}
     # Must exceed WAIT_MINUTES.
     timeout-minutes: 250
     # One line for every arch of every pull request, and only its first job polls.