ci: add job-level timeout-minutes to bound hung post-run cleanup steps

Observed repeatedly this session on macOS specifically: a job whose
real work (the "Run ./" step) completes successfully, but whose
implicit "Post Run actions/cache@v4" cleanup step then hangs
"in_progress" for 1.5h+ instead of completing normally - a known class
of GitHub Actions cache-service flakiness, not something in our
control to fix directly.

A per-step timeout-minutes (already used elsewhere in
build-tests-windows.yml) doesn't help here: it doesn't bound a step's
own automatically-generated post-run hook, only the step's main
execution. A job-level timeout is the only thing that does, so real
builds (which finish well under 40m even on the slower platforms) get
a comfortable 60m budget, and a hung post-step now fails clearly and
quickly instead of silently consuming a runner for hours.

Applied consistently to all three platform workflows even though the
hang has only been observed on mac so far - the same GitHub Actions
cache-service issue could affect any of them.

.github/workflows/build-tests-mac.yml

@@ -12,6 +12,14 @@ jobs:
   buildForAllPlatformsMacOS:
     name: ${{ matrix.targetPlatform }} on ${{ matrix.unityVersion }}
     runs-on: macos-latest
+    # A per-step timeout doesn't bound an action's own implicit post-run
+    # cleanup step (e.g. actions/cache@v4's cache-save), which is exactly
+    # what's been observed hanging here - real builds finish in well under
+    # 40m, but a hung "Post Run actions/cache@v4" step has left jobs stuck
+    # "in_progress" for 1.5h+. A job-level timeout is the only thing that
+    # bounds that too, converting a silent multi-hour hang into a clear,
+    # fast failure.
+    timeout-minutes: 60
     strategy:
       fail-fast: false
       matrix: