Prevent concurrent workflow runs on the same PR

The previous concurrency key mostly did this, but did not consider that
the PR's ref changes when a pull request is merged. In practice, this
meant that closing/merging the PR shortly shortly after updating it
would trigger two workflow runs. Only the latest run (cleanup) should
continue.

.github/workflows/preview_on_downstream_repo.yaml

@@ -13,9 +13,9 @@ on:
         required: true
         type: boolean
 
-# Cancel any in-progress runs of a caller workflow on the same branch.
+# Cancel any in-progress runs of a caller workflow on the same pull request.
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
+  group: ${{ github.workflow }}-${{ github.event.number }}
   cancel-in-progress: true
 
 env: