Trigger debian builds on tags rather than on commits
This aligns our practice with what is expected for upstream debian packages. It also allows package building to become an explicit admin action, e.g. when iterating on a packaging branch locally.
This will also allow reusing the same pipeline for packages where we are not upstream.
Depends on !15 (closed); Depends on !17 (closed).
Test Plan
diff the output of jenkins-jobs and things are sane
Migrated from D1101 (view on Phabricator)