ci: set fake Git user for deploying

Based on the tip at:

- https://github.com/tschaub/gh-pages#deploying-with-github-actions
- tschaub/gh-pages#345 (comment)

.github/workflows/publish-demo.yml

@@ -18,4 +18,4 @@ jobs:
       - name: Configure Git for pushing commits
         # Embed the access token directly into the remote URL so that the gh-pages library can pick it up
         run: git remote set-url origin https://git:${{ secrets.GITHUB_TOKEN }}@github.com/${{ github.repository }}.git
-      - run: npm run deploy
+      - run: npm run deploy -- -u 'github-actions-bot <[email protected]>'