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/systemd-confinement.nix

@@ -150,6 +150,7 @@ import ./make-test-python.nix {
 
     config.users.groups.chroot-testgroup = {};
     config.users.users.chroot-testuser = {
+      isSystemUser = true;
       description = "Chroot Test User";
       group = "chroot-testgroup";
     };