Potentially use rustc-artifacts crate

[Mark-Simulacrum]

https://docs.rs/rustc-artifacts/0.2.0/rustc_artifacts/fn.master_commits.html provides access to bors commit data that doesn't depend on the GitHub API. It provides less information than we might want today, but I would be willing to expand that (I'm the owner of the crate).

That crate currently uses a new triagebot API that lists bors commits made in the last 168 days, currently with just the sha, parent sha, and timestamp of the commit.

That would mean that we can run bisection entirely without the GITHUB_TOKEN or a local rust repository clone, and then use the GitHub API solely to lookup the commit details (if desired).