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.

Signed-off-by: Tomeu Vizoso tomeu.vizoso@collabora.com