Fix cache clearing job (#4868)

On merge github.ref points to master thus attempting to clear cache of master. This commit fixes it with proper referencing + adds write permissions.

Author: vidit-od

.github/workflows/clear-cache.yaml

@@ -9,14 +9,16 @@ on:
 jobs:
   cleanup:
     runs-on: ubuntu-latest
+    permissions:
+      actions: write
     steps:
       - name: Cleanup caches for closed PR
         # remove all cache entries for this PR
         # caches usability hierarchy : master → PR
         # other PRs / master cannot reuse them
         # so clear them when the PR is closed or merged
         run: |
-          gh cache list --ref ${{ github.ref }} --json id \
+          gh cache list --ref refs/pull/${{ github.event.pull_request.number }}/merge --json id \
             | jq -r '.[].id' \
             | xargs -I{} gh cache delete {}
         env: