deploy-report-pages: commits to gh-pages are unsigned

`deploy-report-pages` makes two unsigned commits per run:

1. `peaceiris/actions-gh-pages` — pushes new report files to the `gh-pages` branch
2. A local `git commit` + `git push -f` — commits the cleanup of reports older than N days

Part of #240.

**Complications:**

- `peaceiris/actions-gh-pages` is a third-party action we don't control
- `force_orphan: true` is currently used — it wipes all previous history on each run, which is also incompatible with the cleanup step (cleanup would never find old reports to delete)
- The cleanup step uses `git push -f`
- Report files are base64-encoded binary/HTML — large reports with many screenshots could be a concern depending on the approach taken