(RE-902) Remove sid from build_defaults

Sid is just too unstable.  It is broken upstream often enough to cause
problems in our automated testing, and since we aren't actively testing
Debian Unstable, keeping it in place for every package pipeline run is
not helpful.
Merge branch 'maint/stable/remove-sid-from-build-targets' into stable

* maint/stable/remove-sid-from-build-targets:
  (RE-902) Remove sid from build_defaults