ci(workflows): assign explicit permissions (#168)

Author: caugner

.github/workflows/auto-merge.yml

@@ -3,6 +3,9 @@ name: auto-merge
 on:
   pull_request_target:
 
+# No GITHUB_TOKEN permissions, as we use AUTOMERGE_TOKEN instead.
+permissions: {}
+
 jobs:
   auto-merge:
     uses: mdn/workflows/.github/workflows/auto-merge.yml@main