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-ubuntu.yml

@@ -37,6 +37,10 @@ jobs:
   buildForAllPlatformsUbuntu:
     name: "${{ matrix.targetPlatform }} on ${{ matrix.unityVersion}}${{startsWith(matrix.buildProfile, 'Assets') && ' (via Build Profile)' || '' }}"
     runs-on: ubuntu-latest
+    # See build-tests-mac.yml's matching comment - a per-step timeout doesn't
+    # bound an action's own implicit post-run cleanup step, only a job-level
+    # timeout does.
+    timeout-minutes: 60
     # Known, disclosed gap (see #844's description and src/build-args.ts's own
     # header comment): the CLI always detects the Unity version from the
     # checked-out project's ProjectSettings/ProjectVersion.txt and has no flag