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/ci_ubuntu.yaml

@@ -2,6 +2,9 @@ name: CI - Ubuntu
 
 on: [pull_request, push]
 
+permissions:
+  actions: write
+
 concurrency:
   group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
   cancel-in-progress: ${{ github.head_ref != '' }}