Cross platform version of Julia Evan's flake from "Some notes on nix flakes"

flake.nix

{
  inputs.nixpkgs.url = "nixpkgs/nixos-23.05";
  outputs = { self, nixpkgs }:
    let
      supportedSystems = [ "x86_64-linux" "x86_64-darwin" "aarch64-linux" "aarch64-darwin" ];
      # Helper function to generate an attrset '{ x86_64-linux = f "x86_64-linux"; ... }'.
      forAllSystems = nixpkgs.lib.genAttrs supportedSystems;

      # Nixpkgs instantiated for supported system types.
      nixpkgsFor = forAllSystems (system: import nixpkgs { inherit system; });
    in
    {
    defaultPackage = forAllSystems (system:
      let
        pkgs = nixpkgs.legacyPackages.${system};
      in
      nixpkgs.legacyPackages.${system}.buildEnv {
        name = "homepkgs";
        paths = with pkgs; [
          cowsay
          ruby
        ];
        pathsToLink = [ "/share/man" "/share/doc" "/bin" "/lib" ];
        extraOutputsToInstall = [ "man" "doc" ];
        }
      );
  };
}