This repository exists in two locations in the tree: infra/proto, and chromite/infra/proto. The infra/proto repository is always at ToT, while the chromite/infra/proto checkout is branched to allow the proto there to (more) accurately reflect the version of the proto the Build API is using.
When making changes to the proto that you need to test in the Build API, you will need ensure the changes are applied to the chromite/infra/proto checkout. Chromite generates its proto bindings from the chromite/infra/proto repo.