cleanup-cache: Run gh extension install with GitHub token

The gh CLI always complains in a GitHub Actions workflow without a token.

Author: auscompgeek

.github/workflows/cleanup-cache.yml

@@ -10,10 +10,10 @@ jobs:
   cleanup:
     runs-on: ubuntu-latest
     steps:
-      - run: gh extension install actions/gh-actions-cache
-
       - name: Cleanup
         run: |
+          gh extension install actions/gh-actions-cache
+
           cacheKeysForPR=$(gh actions-cache list -R $REPO -B $BRANCH -L 100 | cut -f 1)
 
           # Turn off errexit to not fail the workflow while deleting cache keys.