Auto-select the concrete helper image flavor on Kubernetes
What does this MR do?
Sets Concrete from FF_CONCRETE in the kubernetes executor's helper-image config, mirroring the docker executor (executors/docker/docker.go).
Why?
Without it, concrete-mode pods bootstrap from the standard helper image. steps bootstrap conditionally copies /git and /ca-certs.pem into the scripts emptyDir — and only the concrete flavor contains them — so with the standard image those copies are silently skipped and concrete's get_sources becomes dependent on git being present in the user's build image. That breaks the parity guarantee the legacy path provides via its helper container (which always has git), and it does so silently.
An explicit helper_image / helper_image_flavor override still takes precedence, unchanged.
Found while documenting helper-image selection for GitLab Functions (!7100): the docker/kubernetes asymmetry looked deliberate at first, but tracing the bootstrap's conditional copies showed it's a gap.
Relates to &21165 (closed).