The listen loop transcribes a check_silence() batch serially, so a later
utterance read its binding after a /voice join landed during an earlier
utterance's STT and was posted to the new channel. Capture the binding
once per batch (and per leave flush) and pass it in.
(cherry picked from commit e4b020a0fd21833a89fc1fa7f904db489d797d9c)
fb49ef18f5