ci-package-builder: Use a full ref for $UPSTREAM_RELEASE
Using a full git ref for $UPSTREAM_RELEASE
rather than just a Debian
suite name offer a greater degree of control when kicking the pipeline
externally, setting the CI variables explicitly.
This is also going to be useful for downstreams that will reuse the
merge-updates
job, mergin from apertis/*
branches rather than the
debian/*
ones.