ci-package-builder: Fix snapshot upload

Commit 31726673 "Automatically tag and upload if the changelog does not say UNRELEASED" broke non-release uploads since the merged upload job only sourced artifacts from the tag-release job.

To fix that, add the build-source in the dependencies: with tag-release and disambiguate the _build directories from the two stages to avoid confusing ci-obs-upload by having two .dsc files.