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.

nixos/modules/config/update-users-groups.pl

@@ -52,8 +52,8 @@ foreach my $g (@{$spec->{groups}}) {
     $gidsUsed{$g->{gid}} = 1 if defined $g->{gid};
 }
 
-foreach my $u (@{$spec->{groups}}) {
-    $uidsUsed{$u->{u}} = 1 if defined $u->{uid};
+foreach my $u (@{$spec->{users}}) {
+    $uidsUsed{$u->{uid}} = 1 if defined $u->{uid};
 }
 
 # Read the current /etc/group.