GHA CI: use least permission level

`actions: write` is required by Chocobo1/setup-ccache-action.
`pull-requests: write` is required by actions/stale.
https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#permissions

.github/workflows/stale_bot.yaml

@@ -4,6 +4,9 @@ on:
   schedule:
     - cron: '0 0 * * *'
 
+permissions:
+  pull-requests: write
+
 jobs:
   stale:
     runs-on: ubuntu-latest