adr: stop using bka for backend tests
Created by: jhchabran
Self-explanatory. Fixes https://github.com/sourcegraph/sourcegraph/issues/35638
Discussion prior to the decision: https://docs.google.com/document/d/1n0WdwtvXXaB7bYtrXQav_-vvs6go21bcpoNDvvYyRRQ/edit#
Test plan
ADR so n/a