fix(workflow): compute branch name from pull_request number (#175)

Author: trivikr

.github/workflows/cache-cleanup-branch.yml

@@ -17,7 +17,7 @@ jobs:
           gh extension install actions/gh-actions-cache
 
           REPO=${{ github.repository }}
-          BRANCH=${{ github.head_ref }}
+          BRANCH="refs/pull/${{ github.event.pull_request.number }}/merge"
 
           echo "Fetching list of cache key"
           cacheKeysForPR=$(gh actions-cache list -R $REPO -B $BRANCH | cut -f 1 )