nixos/users: require one of users.users.name.{isSystemUser,isNormalUser}

As the only consequence of isSystemUser is that if the uid is null then
it's allocated below 500, if a user has uid = something below 500 then
we don't require isSystemUser to be set.

Motivation: https://github.com/NixOS/nixpkgs/issues/112647

nixos/modules/services/misc/nix-daemon.nix

@@ -21,6 +21,7 @@ let
          calls in `libstore/build.cc', don't add any supplementary group
          here except "nixbld".  */
       uid = builtins.add config.ids.uids.nixbld nr;
+      isSystemUser = true;
       group = "nixbld";
       extraGroups = [ "nixbld" ];
     };