flake.nix

{
  outputs = { self, nixpkgs }:
  let system = "x86_64-linux";
  pkgs = import nixpkgs { inherit system; };
  in
  {
    # while this works (nix run)
    packages.${system}.default = pkgs.writeShellScriptBin "hello" "echo bla";

    # this wont build try with nix run
    # and in repl
# nix-repl> builtins.currentSystem
# "x86_64-linux"
    #packages.${system}.default = pkgs.writeShellScriptBin "hello" "echo ${builtins.currentSystem}";
  };
}