Decouple omnibus-build downstream trigger job from package-and-test pipeline

trigger-omnibus now is also used as a validation tool for dependency updates. Given it's not exclusive to package-and-test workflow anymore, it should be extracted outside of child pipeline.

This would also allow more flexibility in setting up rules where we don't allow for omnibus build to fail. Currently it requires some workarounds that make test failures less visible.

It would also slightly simplify setup of .trigger-omnibus-env job as right now it has some duplication due to it being inside a child pipeline template which is supposed to be reusable.