gitlab-ci/lava: Fix image to use in test jobs

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 <tomeu.vizoso@collabora.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>