ci: disentangle tags for containers and artifacts produced by them

In order to reduce the amount of building work and network traffic, we use docker caching. For that, we use the MESA_IMAGE_TAG and MESA_BASE_TAG env variables which build the MESA_IMAGE variable to identify different containers.

We are also using these tags to identify the cached artifacts produced by other containers when those are part of the underlying OS to run directly in DUTs through the DISTRIBUTION_TAG env variable.

The undesirable collateral effect is that we cannot combine a test job using a container which would like to make use of some of the cached artifacts created by another container. In other words, we cannot have a job using a DISTRIBUTION_TAG and a MESA_IMAGE using a different MESA_[IMAGE|BASE]_TAG variables.

Now, we split the usage in the DISTRIBUTION_TAG through the definition of MESA_ARTIFACTS_TAG AND MESA_ARTIFACTS_BASE_TAG.

Signed-off-by: Andres Gomez agomez@igalia.com