nixos/wivrn: Don't run service for system users

When using sddm, the service gets started as the sddm user when
the login screen appears, then is started again as your user after login
while the other is potentially still running. This causes the server
to exit with an 'Address already in use' error in the log.

(cherry picked from commit ad583bb258)

nixos/modules/services/video/wivrn.nix

@@ -201,6 +201,8 @@ in
           # Exec path isn't absolute, it will be resolved relative to $PATH, so we must
           # not override the value of $PATH.
           enableDefaultPath = false;
+
+          unitConfig.ConditionUser = "!@system";
           serviceConfig = (
             if cfg.highPriority then
               {