svga/ci: update EXTERNAL_KERNEL_TAG to new kernel

Update EXTERNAL_KERNEL_TAG to new SHA. Still the same linux 6.8, just stop
consuming kernel artifacts from mesa-lava, instead start using mesa-rootfs

Signed-off-by: Martin Krastev <martin.krastev@broadcom.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34177>
1 file changed