ci: set default permissions for workflows

The default workflow permissions are overly broad; setting permissions
explicitly at the workflow level ensures excessive permissions are not
unintentionally granted to jobs. For details, see:
https://docs.zizmor.sh/audits/#excessive-permissions

.github/workflows/optional.yml

@@ -4,6 +4,9 @@ on:
     types: [labeled, opened, synchronize, reopened]
   workflow_dispatch:
 
+permissions:
+  contents: read
+
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
   cancel-in-progress: ${{ github.event_name == 'pull_request' }}