Use !cancelled() in required-status-check

When the build is cancelled by concurrency (e.g. a 'test *' label
triggers a rebuild via rebuild-pull-request-on-label.yml), the
superseded run's required-status-check used to run with always() and
report failure (because needs.*.result was 'cancelled'). Branch
protection then showed the PR as red until the rebuild completed.

Switching to !cancelled() makes the superseded check-run be reported
as skipped instead, and branch protection resolves the same-named
check from the rebuild.

Author: trask

.github/workflows/build-pull-request.yml

@@ -69,7 +69,13 @@ jobs:
       - muzzle
       - markdown-lint-check
     runs-on: ubuntu-latest
-    if: always()
+    # `!cancelled()` (instead of `always()`) makes this a "skipped" check-run
+    # when the workflow is cancelled by concurrency (e.g. when a "test *"
+    # label triggers a rebuild via rebuild-pull-request-on-label.yml). That
+    # avoids a noisy red "required-status-check failed" on the superseded
+    # run; the rebuild produces a same-named check that branch protection
+    # resolves to success.
+    if: ${{ !cancelled() }}
     steps:
       - if: |
           needs.common.result != 'success' ||