update-users-groups.pl: correctly guard duplicate uids for declarative users

Verified that following nixos configuration:

    users.users.foo = {
      uid = 1000;
      name = "foo";
    };
    users.users.bar = {
      name = "bar";
    };

Before this commit both users will get uid of 1000, after it's applied
bar will correctly get 1001.