[BASOP-CI] fix name detection script for merges to main-pc

In BASOP repo pipelines targeting main-pc, the float reference branch name is gotten via a script which checks if the needed branch exists by matching on it over the list of branches. This listing of the branches was buggy: git branch -av was used and the -v switch includes all the last commit messages of the branches in the output (dunno really why I used that in the first place..). This can cause an error if the name of the needed branch is mentioned in a commit/merge message.

Fix is to only use -a switch.