gitlab-ci: More stable URL for kernel and ramdisk artifacts, for LAVA

gitlab-ci: More stable URL for kernel and ramdisk artifacts, for LAVA

Place the kernel and ramdisk into a place in the file server so the URL
will only change when the contents also change.

Also put the Mesa build into a separate tarball so the ramdisk's
contents don't change every build.

With proper caching in place, all devices in the same farm need only to
download the mesa tarball once, saving time.

As we switch to MinIO for making kernels and rootfs available to LAVA
devices, we can stop using Docker to distribute them.

Instead, build when needed in separate jobs that push directly to MinIO,
from where LAVA devices can download them.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5515>