nixos-18.03/pgbackup: Fix bug in postgresql-backup module that causes

pg_dump to fail

Run pg_dump as postgres super user instead of root.
See https://github.com/NixOS/nixpkgs/issues/41388

nixos/modules/services/backup/postgresql-backup.nix

@@ -9,7 +9,7 @@ let
 
   postgresqlBackupCron = db:
     ''
-      ${config.services.postgresqlBackup.period} root ${config.services.postgresql.package}/bin/pg_dump ${db} | ${gzip}/bin/gzip -c > ${location}/${db}.gz
+      ${config.services.postgresqlBackup.period} root ${config.services.postgresql.package}/bin/pg_dump ${db} -U postgres | ${gzip}/bin/gzip -c > ${location}/${db}.gz
     '';
 
 in