ci: Switch the baremetal runner to be an x86 docker image.

ci: Switch the baremetal runner to be an x86 docker image.

The runner is an x86 system, so running the ARM image meant doing
everything at runtime under qemu, and for the xz of the test rootfs that
was quite expensive.  Also, we can rebuild x86 images much faster than we
can rebuild arm images for container development, which will help unblock
some of the other feature parity work I have to do versus the old docker
system that cheza is using.

Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5033>