Update CleanUpCache workflow (#3403)

The pr_number artifact is only available for `pull_request`. The logic
there will become incorrect if another event like `schedule` is added,
because it only assumes it's either `push` or `pull_request`.

.github/workflows/cleanup-cache.yml

@@ -23,18 +23,18 @@ jobs:
 
           REPO=${{ github.repository }}
 
-          # push or pull_request
+          # push or pull_request or schedule or ...
           EVENT=${{ github.event.workflow_run.event }}
 
           # Triggering workflow run name (e.g., LinuxClang)
           WORKFLOW_NAME=${{ github.event.workflow_run.name }}
 
-          if [[ $EVENT == "push" ]]; then
-            BRANCH=refs/heads/${{ github.event.workflow_run.head_branch }}
-          else
+          if [[ $EVENT == "pull_request" ]]; then
             gh run download ${{ github.event.workflow_run.id }} -n pr_number
             pr_number=`cat pr_number.txt`
             BRANCH=refs/pull/${pr_number}/merge
+          else
+            BRANCH=refs/heads/${{ github.event.workflow_run.head_branch }}
           fi
 
           # Setting this to not fail the workflow while deleting cache keys.