Pair Cargo releases with Rust releases

[brson]

While Cargo does have tagged releases, we don't actually use those in the Rust releases, instead pairing rustc with an arbitrary recent revision of Cargo.

Update rust-packaging to pair rustc not with Cargo nightlies, but with Cargo stable releases.

This will probably require having 'stable' cargo dist bots that produce versioned artifacts, though right now we're solving this in a hackish way by only tagging cargo commits that were nightlies.

re https://internals.rust-lang.org/t/perfecting-rust-packaging-the-plan/2767

[carols10cents]

I think this can be closed now since cargo rides the trains...