make-disk-image: use OVMF efi always

[MatthewCroughan]

When building disk images, the VM used to perform partitioning/mkfs inherits the kernel from the nixosConfiguration we're trying to build for. This works fine, until you try to boot a kernel with zboot, and in that case you need UEFI to load the image, otherwise you end up with the following log from QEMU:

"unable to handle EFI zboot image with ... compression"

https://github.com/qemu/qemu/blob/master/hw/core/loader.c#L924

Loading OVMF allows us to bypass this issue, and boot the kernel successfully like this:

corpo-disko-images> UEFI firmware (version  built at 00:00:00 on Jan  1 1980)
corpo-disko-images> EFI stub: Decompressing Linux Kernel...
corpo-disko-images> EFI stub: Loaded initrd from LINUX_EFI_INITRD_MEDIA_GUID device path
corpo-disko-images> EFI stub: Generating empty DTB
corpo-disko-images> EFI stub: Exiting boot services...
corpo-disko-images> loading kernel modules...

I think the architecture of inheriting the host kernel just to make a disk image from a nixosConfiguration might still be a bit wrong. But this PR fixes the issue for the time being.

[Mic92]

I made an amendment and also made efi vars writable: #660