gitlab-ci: Consolidate container and build stages for LAVA

gitlab-ci: Consolidate container and build stages for LAVA

Use the normal build job to also prepare the artifacts for LAVA jobs.

For that, the build container needs to also build the test suites,
kernel, ramdisk, etc.

Then the build job will place the just-built Mesa in the ramdisk and the
test job can generate a LAVA job and point to those artifacts.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3295>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3295>