Fixed wrong name of environmental variable

Author: hsbt

.github/workflows/dependabot_automerge.yml

@@ -29,4 +29,4 @@ jobs:
         run: gh pr merge --auto --merge "$PR_URL"
         env:
           PR_URL: ${{ github.event.pull_request.html_url }}
-          GITHUB_TOKEN: ${{ secrets.MATZBOT_GITHUB_TOKEN }}
+          GH_TOKEN: ${{ secrets.MATZBOT_GITHUB_TOKEN }}