transcriberbin: fix initial transcription bin setup
When passthrough=false at construction and the transcription bin is linked after receiving video caps (and not on state change), there could be a race where transcription-bin was linked with tee but state change of the transcription-bin was not finished.
If upstream pushed a buffer at that point, it got a flushing flow return and stopped streaming.
This is the same issue and the same fix as 558656de for the initial passthrough=false case.