Update KOKKURI_BLUBBER_IMAGE to point to repos/releng/blubber

The blubber buildkit image build via GitLab is published to a different location than w/ the old pipelinelib.

The new path under docker-registry.wikimedia.org is "/repos/releng/blubber" (formerly "/wikimedia/blubber-buildkit"). It would be nice if we could publish an image from GitLab with "buildkit" in its name but maybe we can figure that issue out later (possibly by renaming the repo).

Also, advance to blubber v0.12.1.