Make RustRover work with Rust from Nixpkgs

Force RustRover to semi-work with Rust installations from Nixpkgs#

Works by creating a toolchain in your .idea folder. Tested with a toolchain from Nixpkgs, see examples/nixpkgs.nix. Probably also works with fenix, see examples/fenix.nix.

TODO:

  • An example of using in Flakes (without relying on toString ./. being the impure project root)