Give cache-cleaning workflow permissions to access caches (#256)

- **Add workflow to delete cache entry upon PR merge**
- **Give permission**

.github/workflows/cleanup-pr.yml

@@ -7,6 +7,8 @@ on:
 jobs:
   cleanup:
     runs-on: ubuntu-latest
+    permissions:
+      actions: write
     steps:
       - name: Cleanup
         run: |