mariabackup --ftwrl-wait-timeout never times out on explicit lock

Mariabackup --ftwrl-wait-timeout only times out if FLUSH is blocked by an actual running query in another session; same for BACKUP LOGS in 10.4.

If FLUSH is blocked by an explicit LOCK TABLES ... WRITE being active in another session though, which is otherwise idle, mariabackup will just continue waiting forever, instead of giving up after having reached the timeout ...