grub-installer: no-nvram for arm64

I'm forwarding this patch from https://github.com/aarch64-laptops/debian-cdimage/blob/main/patches/debian-installer/grub-installer/0001-grub-installer-no-nvram-for-arm64.patch

The goal is to improve support of arm64 laptops. I don't know much about the rationale for the change but I'll try to gather this with a subsequent mails to all involved parties.