bluetooth/gst: Only retrieve sink pad once

This causes pretty verbose logs otherwise, and is unnecessarily
expensive.
3 jobs for gst-synchronous in 8 minutes and 11 seconds (queued for 1 second)
Status Job ID Name Coverage
  Container
passed #7029279
build-container

00:00:14

 
  Build
passed #7029280
build-autotools

00:07:56

passed #7029281
build-meson

00:03:17