Rust components not available when rust targets are specified

[wvffle]

Describe the bug
When I specify the targets for rust language, the rust components like cargo are never installed. When I specify channel that is other than nixpkgs, then the components are there.

To reproduce

# flake.nix
{
  description = "...";

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

    flake-utils.url = "github:numtide/flake-utils";
    flake-utils.inputs.nixpkgs.follows = "nixpkgs";

    devenv.url = "github:cachix/devenv";
    devenv.inputs.nixpkgs.follows = "nixpkgs";

    flake-parts.url = "github:hercules-ci/flake-parts";
    flake-parts.inputs.nixpkgs.follows = "nixpkgs";
  };

  outputs = inputs@{ flake-parts, nixpkgs, ... }:
    flake-parts.lib.mkFlake { inherit inputs; } {
      imports = [ inputs.devenv.flakeModule ];
      systems = nixpkgs.lib.systems.flakeExposed;

      perSystem = { config, pkgs, ... }: {
        devenv.shells.default = {
          languages.rust = {
            enable = true;
            targets = [ "wasm32-unknown-unknown" ];
          };
        };
      };
    };
}

When I try to check if cargo is present:

│❯❯❯ cargo                                                                                               ✘ 1  │
│The program 'cargo' is not in your PATH. It is provided by several packages.                                 │
│You can make it available in an ephemeral shell by typing one of the following:                              │
│  nix-shell -p cargo                                                                                         │
│  nix-shell -p cargo-auditable-cargo-wrapper                                                                 │
│  nix-shell -p rustup  

Version
devenv: 1.0.5

[sandydoo]

You have to use the fenix toolchain when cross-compiling. I don't think there's tooling to do that within nixpkgs, but I could be wrong.
We'll add an error message for this case.