Fix dependabot auto-merge by using GH_TOKEN env var

The gh CLI in GitHub Actions no longer reads GITHUB_TOKEN and
requires GH_TOKEN instead, causing the merge step to exit with code 4.

Author: k0kubun

.github/workflows/dependabot_automerge.yml

@@ -27,4 +27,4 @@ jobs:
         run: gh pr merge --auto --rebase "$PR_URL"
         env:
           PR_URL: ${{ github.event.pull_request.html_url }}
-          GITHUB_TOKEN: ${{ secrets.MATZBOT_DEPENDABOT_MERGE_TOKEN }}
+          GH_TOKEN: ${{ secrets.MATZBOT_DEPENDABOT_MERGE_TOKEN }}