Fetch Cargo manually from new archives

[rust-highfive]

r? @brson

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[alexcrichton]

The tl;dr; of the scheme I'm envisioning is:

• Cargo has a bunch of rust-1.X.Y branches
• Rust version 1.X.Y is paired with the Cargo produced from the branch rust-1.X.Y
• When you r+ a PR on Cargo, it uploads all artifacts that were built and tested
• As a result, all merge commits on the rust-1.X.Y are guaranteed to have artifacts uploaded
• Backporting is therefore just a PR against rust-1.X.Y with a @bors: r+
• On this, the packaging side, we just pick up the latest commit on rust-1.X.Y with an API call to GitHub, and then we download the right artifact from there.

I haven't quite figure out how to deal with Cargo's versioning just yet. Cargo builds all release artifacts on the auto-cargo branch and doesn't have a great way of distinguishing what channel the build is for. Similarly, if we have different names for artifacts in the cargo upload directories we'd need to discover that here as well.

What I envision is that all rust-1.X.Y branches in Cargo are built with --release-channel=stable for Cargo. The artifacts are then named with cargo-<cargo version>-<target>.tar.gz. I guess we can upload some file in each folder with a deterministic name with the contents being <cargo version>?

@brson, thoughts on this?

[brson]

This all sounds fine to me. r+