On certain environments, grub2-install fails trying to update nvram,
even when it should not be updating it anyway.
This is a temporary measure anyway, the longer term plan is to avoid
calling grub2-install in UEFI environments.

Before:
Using GRUB2.
Writing grub /var/tmp/imgcreate-_9hsu5qa/install_root/boot/grub2/device.map
Installing grub2 to /dev/loop0
Installing for arm64-efi platform.
Could not prepare Boot variable: No such file or directory
grub2-install: error: efibootmgr failed to register the boot entry: Input/output error.
Unable to create appliance : Unable to install grub2 bootloader

After:
Using GRUB2.
Writing grub /var/tmp/imgcreate-5r27_2zx/install_root/boot/grub2/device.map
Installing grub2 to /dev/loop0
Installing for arm64-efi platform.
Installation finished. No error reported.

Signed-off-by: Pablo Greco pgreco@centosproject.org