ci: Build ARM baremetal rootfs in native container

ci: Build ARM baremetal rootfs in native container

Doing so in an x86 container via qemu was slow, and started failing
recently after updating to a newer qemu version.

This also results in smaller arm*_test* docker images, since we need to
install fewer Debian packages in them.

As a bonus, this turns some piglit tests from fail to pass (Or maybe
they'll turn out to be flakes? They've passed at least 3 times in a
row).

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9955>