boards/x86: scripts: unset CFLAGS when running build_grub.sh

On Clear Linux, the CFLAGS is set and defines a number of aggressive
checks and optimizations. This causes a build failure when generating a
GRUB2 boot loader image using the 'build_grub.sh' script.

Unsetting it within the script allows it to proceed and successfully
build a functional GRUB2 boot loader image to be used with Zephyr.

Fixes: #14289
Signed-off-by: Geoffroy Van Cutsem <geoffroy.vancutsem@intel.com>