x-gpt-efi-disk.wks.in: Use the grub.cfg from deploy directory

The grub.cfg file is processed based on the machine configuration and
the final result is deployed in DEPLOY_DIR_IMAGE. Use that file as
opposed to letting wic finding it in the layer - and using the template
one.

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>