nixos/libeufin: fix postgresql.package regression

`config.services.postgresql.package` is now only available when the
PostgreSQL module is actually enabled. If we're not using the local
database, we'll need to fallback to the latest postgresql version, since
we don't know about the remote version.

nixos/modules/services/finance/libeufin/common.nix

@@ -96,7 +96,9 @@ libeufinComponent:
             };
           in
           {
-            path = [ config.services.postgresql.package ];
+            path = [
+              (if cfg.createLocalDatabase then config.services.postgresql.package else pkgs.postgresql)
+            ];
             serviceConfig = {
               Type = "oneshot";
               DynamicUser = true;