Fix merging of system.defaults.CustomUserPreferences

[zhaofengli]

Without this, custom user preferences defined in different modules for the same domain aren't merged.

For example, the following config results in only UniversalSearchEnabled defined:

{
  imports = [
    {
      system.defaults.CustomUserPreferences = {
        "com.apple.Safari"."NSUserKeyEquivalents"."Quit Safari" = "@~q";
      };
    }
    {
      system.defaults.CustomUserPreferences = {
        "com.apple.Safari"."UniversalSearchEnabled" = false;
      };
    }
  ];
}

[Enzime]

LGTM

Thanks for the contribution 🎉