Use latest docker-dind/docker-builder tags for build jobs

Build jobs were using $CI_COMMIT_SHA as the image tag, but when
build_docker_dind_image is skipped (because Dockerfile.docker-dind
wasn't changed), the jobs fail trying to pull non-existent images.

Fixed by setting DOCKER_DIND_TAG and DOCKER_BUILDER_TAG to "latest"
for all build jobs that don't depend on fresh docker-dind/builder builds.