Revert "chore: disabling PR checks for GHA unless they are merged to main (#1342) (#1343)"

This reverts commit 585607c.

Author: NeerajDwivedii

.github/workflows/test-runner.yml

@@ -1,5 +1,7 @@
 name: "gha: macOS & Windows"
 
+# Build on pull requests and pushes to `main`. The PR builds will be
+# non-blocking for now, but that is configured elsewhere.
 on:
   # Only trigger on actual pushes to these branches.
   # A PR merge counts as a push to the target branch.
@@ -18,7 +20,7 @@ on:
           default: true
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
+  group: ${{ github.workflow }}-${{ github.event.pull_request.head.label || github.head_ref || github.ref }}
   cancel-in-progress: true
 
 jobs: