also skip proxmox-grub if not booted in EFI mode

also skip proxmox-grub if not booted in EFI mode

This would effectively pull in grub-efi-amd64, which we skip a line
above this if not in EFI mode..

The builder now adds this always to the packages due to the
proxmox-secure-boot-support meta package being present there, at least
that's my current educated guess (confirmed in practice).

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>