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/tests/shadow.nix

@@ -13,14 +13,17 @@ in import ./make-test-python.nix ({ pkgs, ... }: {
     users = {
       mutableUsers = true;
       users.emma = {
+        isNormalUser = true;
         password = password1;
         shell = pkgs.bash;
       };
       users.layla = {
+        isNormalUser = true;
         password = password2;
         shell = pkgs.shadow;
       };
       users.ash = {
+        isNormalUser = true;
         password = password4;
         shell = pkgs.bash;
       };