Add a way to select Rust toolchain version

[david7a68]

Is your feature request related to a problem? Please describe.
I'm trying to integrate rules_nixpkgs for a Rust project but have found it difficult to configure it such that the toolchain is derived from the same flake.lock that we use to build our devshell (that is, the toolchain advances in lockstep).

rules_rust does provide an example of how to do this here. However, I find the entire arrangement rather intimidating and, for one dipping their toes into rust/nix/bazel interop, more involved and fragile than I am willing to commit. If I have this issue, I suspect that many other people might also.

Describe the solution you'd like
Some method (or documentation that provides guidance) to select a particular toolchain version (as one does with oxalica's rust-overlay) without copying down a large amount of toolchain configuration code.

The existing example does not suggest how this ought to be done (and indeed, I haven't been able to figure out how to get it off of rustc 1.86.0).

Describe alternatives you've considered
None

Additional context
None