Build unit test & lint images with Blubber

It was no more possible to build and push images to the Docker registry with Kokkuri, using trusted runners, and describing the image with the Dockerfile syntax. Since then, our CI had to rebuild the whole environment at each job run. A very slow and error-prone process.

With this patch we are describing the unit-test & linting images with Blubber. It allows us to pick them for running our regular CI jobs in a precomputed and stable environment, as we used before the limitation. Going from ~6min to 20s, and allowing us to us a larger CI workers pool.

Note: The generation of the deb package still uses a custom worker. But SREs are going to refactor the whole Airflow deployment process when migrating to Kubernetes.

Bug: T351792