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

[academo]

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