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/coverity-scan.yml

@@ -5,6 +5,8 @@ on:
     - cron: '0 0 1 * *' # Monthly (1st day of month at midnight)
   workflow_dispatch: # Mainly for testing. Don't forget the Coverity usage limits.
 
+permissions: {}
+
 jobs:
   coverity_scan:
     name: Scan