Can machines be used to run end to end tests?

Yes, they are.

Doesn’t use machines, but ref code using the fly-pr-review-apps github-action: https://github.com/search?l=YAML&q=“+fly-pr-review-apps”&type=Code

Some developers have chosen to fork as Fly has all but abandoned the effort: superfly / fly-pr-review-apps - anyone maintaining?. Regardless, fly-pr-review-apps is a good starting point for anyone trying to build their own CI workflow atop github-actions.