First live run of the cache trio failed at flag-parse time: the
default buildx `docker` driver hard-errors on cache-to registry
export ('Cache export is not supported for the docker driver'), which
failed build-image on PR #2593 and skipped the entire gate eval
matrix behind it. docker/setup-buildx-action creates the
docker-container builder that supports registry cache export; all
three build sites (evals, evals-periodic, ci-image) get it.