Fix for host env leak into grub

While building images in a dual boot host,
/dev/sda is leaking into build's grub config.
It is visible in the logs that the commands
GRUB-INSTALL and GRUB-MKCONFIG probes Host OS environment.
This is avoided by setting grub_disable_os_prober
to true in /default/grub config file.

Change-Id: I7408de7b3bc8a62f8bf103f00a88a596f988353c
Closes-Bug: 1166491