Fix pretix schema migration issue

Fixes django.db.migrations.exceptions.MigrationSchemaMissing

pkgs/pretix/module.nix

@@ -193,6 +193,7 @@ in
         CREATE ROLE ${postgresql.user} WITH LOGIN PASSWORD '${postgresql.password}' CREATEDB;
         CREATE DATABASE ${postgresql.database};
         GRANT ALL PRIVILEGES ON DATABASE ${postgresql.database} TO ${postgresql.user};
+        ALTER DATABASE ${postgresql.database} OWNER TO ${postgresql.user};
       '';
     };