ci(actions): use different GitHub token for automege updates

Hopefully this fixes the PR run issues with dependency PRs

Author: jrolfs

.github/workflows/automerge.yml

@@ -24,7 +24,7 @@ jobs:
       - name: automerge
         uses: 'pascalgn/automerge-action@f81beb99aef41bb55ad072857d43073fba833a98'
         env:
-          GITHUB_TOKEN: '${{ secrets.GITHUB_TOKEN }}'
+          GITHUB_TOKEN: '${{ secrets.AUTOMERGE_TOKEN }}'
           MERGE_LABELS: 'automerge,!work in progress'
           MERGE_REMOVE_LABELS: 'automerge'
           MERGE_METHOD: 'rebase'