Crater should use the same cargo version to generate lockfiles as to do the build

[aidanhs]

rust-lang/cargo#4563 has made generating with stable and building with beta and --frozen break.

cc @Mark-Simulacrum

[Mark-Simulacrum]

So it's kind of hard to know what we want to do here, as we use two different versions of Cargo for building crates (as we should). I suspect that using a stable Cargo might be the best thing we can do here. Given unstable features in Cargo, though, that imposes some limitations on us that we'd rather not have.