build-env: Guess again target release from branch name

Commit ea6cd2a5 "Fix $APERTIS_RELEASE override wrt. OBS projects" dropped the heuristic to guess the target release from a plain branch name, for instance guessing v2024dev2 from foo-v2024dev2-bar.

However, that ability turned out to be very useful when trying to quickly test build on OBS something that would target a stable release, without having to create 1. a MR and 2. a -updates branch for the stable release, since the freeze logic ensures that MR targeting stable branches directly fail so they cannot be merged.