Instead, discover it automatically when building the package.
This makes `buildRustPackage` more future-proof with respect to changes
in how `cargo` generates the hash.
Also, it fixes broken builds in i686 because apparently, cargo generates
a different registry index hash in this architecture (compared to
x86-64).
This makes buildRustPackage portable to non-Linux platforms.
Additionally, now we also save the `Cargo.lock` file into the fetch output, so
that we don't have to run $cargoUpdateHook again just before building.
I was asked to add this for backwards comparability when I renamed `rust`
to `rustc` and added `rustcMaster`. It has been a few months so I'd hope
deprecating this is acceptable.
