ci: add checkout step to clean-cache workflow

- Add `actions/checkout@v4` as the first step in the cleanup job

Author: nicholas-fedor

.github/workflows/clean-cache.yaml

@@ -14,6 +14,8 @@ jobs:
     if: github.event.pull_request.merged
     runs-on: ubuntu-latest
     steps:
+      - name: Checkout
+        uses: actions/checkout@v4
       - uses: ./.github/actions/clean-cache
         with:
           gh-token: ${{ secrets.GITHUB_TOKEN }}