make-package: Allow to specify the source to build the package for
It sometimes happen that the build is broken on unstable due to a random amount of upstream dependencies failing to build. Thus making our package build impossible to run completely (stable is our current infra target).
This allows for avoiding properly the issue by being able to specify the target to build the package for. And this, without having to locally amend the script.
Migrated from D393 (view on Phabricator)