ci(actions): use a separate label to enable autoupdate of PRs

Author: jrolfs

.github/workflows/automerge.yml

@@ -32,5 +32,5 @@ jobs:
           MERGE_FORKS: 'false'
           MERGE_RETRIES: '6'
           MERGE_RETRY_SLEEP: '10000'
-          UPDATE_LABELS: ''
+          UPDATE_LABELS: 'autoupdate'
           UPDATE_METHOD: 'rebase'