Get the default value of a NixOS option

option.nix

# $ nix-instantiate --eval option.nix --argstr path services.journald.enableHttpGateway
# false
{ path }:
let
  pkgs = import <nixpkgs> { };
  evalConfig = import "${<nixpkgs>}/nixos/lib/eval-config.nix";
  inherit (pkgs) lib;
in (evalConfig {
  modules = [
    ({ config, ... }: {
      options.cookie = lib.mkOption { default = null; };
      config.cookie = lib.getAttrFromPath (lib.splitString "." path) config;
      config._module.check = false;
      config.fileSystems."/".device = "/fake";
      config.boot.loader.grub.devices = [ "/fake" ];
    })
  ];
}).config.cookie