fixup: ttcn3-bts-test: use an older version of fake_trx.py
Setting OSMO_BB_BRANCH=ttcn3-bts-test did not achieve the goal, because it was not consumed by the osmocom-bb-host-master's Makefile. Setting it in jenkins.sh does the trick.
Change-Id: Ib6857fafd4085aae91eec9c67f08766bdabeb2b7 Fixes: 8540ae7 ("ttcn3-bts-test: use an older version of fake_trx.py")
