gitlab-ci/lava: Fix image to use in test jobs
In the test stage, we can use any of the two container images as we arent going to do anything architecture-dependent when submitting the jobs to LAVA.
But if we are in a pipeline in which the images need to be rebuilt and one finishes much earlier than the other, it could happen that the test job that executes first fails to find the container image.
To avoid that, have each job in the test stage to use the image that has been already implicitly built by depending on the build job for the given arch.
Signed-off-by: Tomeu Vizoso email@example.com