Jekyll: Jekyll HEAD has a breaking change that breaks some plugins.

• [x] I believe this to be a bug, not a question about using Jekyll.
• [x] I updated to the latest Jekyll (or) if on GitHub Pages to the latest github-pages
• [x] I read the CONTRIBUTION file at https://jekyllrb.com/docs/contributing/
• [ ] This is a feature request.

• [x] I am on (or have tested on) _macOS_ 10+
• [x] I am on (or have tested on) _Debian/Ubuntu_ GNU/Linux
• [ ] I am on (or have tested on) _Fedora_ GNU/Linux
• [ ] I am on (or have tested on) _Arch_ GNU/Linux
• [ ] I am on (or have tested on) _Other_ GNU/Linux
• [x] I am on (or have tested on) _Windows_ 10+

See: https://github.com/jekyll/jekyll-sitemap/issues/123