> cat /home/name/packages/my-script/default.nix
{ writeShellScriptBin }:
writeShellScriptBin "helloWorld" "echo Hello World";

> cat /etc/nixos/configuration.nix
environment.systemPackages = [ (callPackage /home/name/packages/my-script {}) ];

> cat /home/name/packages/my-script/shell.nix
with import <nixpkgs> { };

let
  myScript = callPackage ./default.nix {};
in stdenv.mkDerivation rec {
  name = "test-environment";

  buildInputs = [ myScript ];
}