sanity-check-manual: need to be run on a system with its rootfs on an eMMC

This test is expected to be run on a system with its rootfs on an eMMC storage
and not on a USB device. Using a rootfs from a USB device can lead to
failures like the systemd service 'usbmount@dev-sda1.service' failing to
start since /dev/sda1 is already mounted at /boot/efi.

Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>