Provide nix-darwin module for nix.custom.conf

[lucperkins]

This module would enable Determinate Nix + nix-darwin users to do this:

{
  # Let Determinate Nix set `/etc/nix/nix.conf`
  nix.enable = false;

  # Set `/etc/nix/nix.custom.conf`
  determinate-nix.customSettings = {
    lazy-trees = true;
  };
}

I currently have this working as expected in my own config:

https://github.com/the-nix-way/nome/blob/main/flake.nix#L142-L180
https://github.com/the-nix-way/nome/blob/main/nix-darwin/base/default.nix#L17-L19