Don't let irrelevant label-adds cancel the in-progress PR build

GitHub evaluates a workflow's `concurrency:` group BEFORE its job-level `if:` filter. When a non-build-affecting label is added to a PR (e.g. `automated code review` from code-review-sweep, any auto-label from .github/labeler.yml, or any human/bot label-add), the resulting `pull_request labeled` run enters the shared per-PR group with cancel-in-progress, cancels the real in-progress build, and only then evaluates the `if:` and skips — leaving the PR with no build and a failing required-status-check (see PR #18441).

Route those irrelevant-label runs to a unique, per-run-id throwaway concurrency group so they neither cancel nor are cancelled by anything; the job's `if:` then skips them in ~1s. Build-affecting labels (`test native`, `test openj9`, `test windows`) and all other event types keep the shared per-PR group, preserving the existing cancel-and-rerun semantics. The fix is at the build workflow level, so it applies uniformly to every label-add source (humans, the labeler workflow, otelbot, dependabot, etc.).

Author: trask

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

@@ -8,8 +8,29 @@ on:
       - reopened
       - labeled
 
+# When a `labeled` event adds a label that doesn't affect the build (see
+# the `if:` on the build job below), route it to a unique, throwaway
+# concurrency group so it neither cancels nor is cancelled by anything.
+# Otherwise it would enter the shared per-PR group, cancel the real
+# in-progress build (because concurrency is evaluated before the job-level
+# `if:`), and then skip — leaving the PR with no build. This affects any
+# label-add source: humans, the `labeler.yml` auto-labeler, the
+# code-review-sweep otelbot app token, dependabot, etc.
+#
+# Build-affecting labels (`test native` / `test openj9` / `test windows`)
+# and all other event types keep the shared per-PR group with
+# cancel-in-progress, so a new push or a new test-label still cancels and
+# supersedes any in-progress build.
 concurrency:
-  group: build-pull-request-${{ github.event.pull_request.number }}
+  group: >-
+    ${{
+      (github.event.action == 'labeled' &&
+       github.event.label.name != 'test native' &&
+       github.event.label.name != 'test openj9' &&
+       github.event.label.name != 'test windows')
+      && format('build-pull-request-noop-{0}', github.run_id)
+      || format('build-pull-request-{0}', github.event.pull_request.number)
+    }}
   cancel-in-progress: true
 
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