nixos/gitlab: fix gitlab-registry-cert path condition

`ConditionPathExists` belongs in the [Unit] section, not [Service].
The unit now properly checks if the cert file already
exists before activating so certs will not be overwritten anymore.

nixos/modules/services/misc/gitlab.nix

@@ -1043,7 +1043,7 @@ in {
         chown ${cfg.user}:${cfg.group} ${cfg.registry.certFile}
       '';
 
-      serviceConfig = {
+      unitConfig = {
         ConditionPathExists = "!${cfg.registry.certFile}";
       };
     };