Minimize flake

[ada-x64]

I am currently working on a nix-based Bevy ecosystem and am pushing out a minimized toolchain. Right now, the bevy_cli flake is packaging the default profile for Rust. This PR changes it to a minimal profile.

Example of downstream usage from https://github.com/cubething-qproj/infra

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";

    rust-overlay = {
      url = "github:oxalica/rust-overlay";
      inputs.nixpkgs.follows = "nixpkgs";
    };

    bevy_cli = {
      url = "github:TheBevyFlock/bevy_cli";
      inputs.nixpkgs.follows = "nixpkgs";
      inputs.rust-overlay.follows = "rust-overlay";
    };
  };

  outputs =
    {
      nixpkgs,
      rust-overlay,
      bevy_cli,
      ...
    }:
    let
      system = "x86_64-linux";
      pkgs = import nixpkgs {
        inherit system;
        overlays = [ rust-overlay.overlays.default ];
      };

      # Discover the exact nightly required by bevy_lint without
      # independently hardcoding it downstream.
      rustChannel =
        bevy_cli.packages.${system}.bevy.rustToolchainChannel;

      # The packaged CLI retains only its minimal rustc-private sysroot.
      bevy-cli = bevy_cli.packages.${system}.bevy;
    in
    {
      devShells.${system}.default = pkgs.mkShell {
        packages = [
          bevy-cli
          # The project's own Rust toolchain and other development tools.
        ];
      };
    };
}

The important part is that both upstream inputs follow the downstream versions:

  bevy_cli.inputs.nixpkgs.follows = "nixpkgs";
  bevy_cli.inputs.rust-overlay.follows = "rust-overlay";

This lets Nix deduplicate the underlying rustc, cargo, rust-std, and rustc-dev component paths even when the project has its own dev toolchain.

[DaAlbrecht]

Hi Thanks for this PR. I dont use nix so i cant really review this, maybe @musjj could help?