fix(modules/dgn-console): Only try to create the psql migrate script if postgres is enabled

modules/dgn-console.nix

@@ -71,7 +71,8 @@ in {
     hardware.enableRedistributableFirmware = true;
 
     environment.systemPackages = (with pkgs; [ neovim wget kitty.terminfo ])
-      ++ lib.optional (config.services.postgresql.package != cfg.pg-upgrade-to)
+      ++ lib.optional (config.services.postgresql.enable
+        && config.services.postgresql.package != cfg.pg-upgrade-to)
       (pkgs.writeScriptBin "upgrade-pg-cluster" ''
         set -eux
         # XXX it's perhaps advisable to stop all services that depend on postgresql