nixos/mastodon: make robust for cases where postgresql isn't enabled on the host

E.g. when a remote PostgreSQL database is used

Apparently you can't access the services.postgresql.package value
anymore if postgresql isn't enabled?

nixos/modules/services/web-apps/mastodon.nix

···
             '';
           path = [
             cfg.package
-            config.services.postgresql.package
+            (if databaseActuallyCreateLocally then config.services.postgresql.package else pkgs.postgresql)
           ];
           environment =
             env