Set a option with its default value in nixos configuration

[t184256]

@Vonfry

I can get default value from website or nixpkgs repo.
I wonder how I can get it using nix expression in configuration.nix. I know it is lazy in config, so using argument named config cannot work.

As an alternative, more literal approach, this could work:

{ config, options, ... }:
{ nix.nixPath = options.nix.nixPath.default ++ [sth]; }