Fix $APERTIS_RELEASE override wrt. OBS projects

Setting $APERTIS_RELEASE when triggering a pipeline used to override the target autodetection, but since commit 06f2a417 when computing the target OBS project it was no longer taken in account.

This commit cleans up the autodetection a bit and lets people set things like APERTIS_RELEASE=v2022-updates to properly override the pipeline destination project.