fix: Use and configure grub for mirrored efi partitions

[provokateurin]

This works on my machine, but for some odd reason grub-install fails when running the check for the example:

       > machine # Installing for i386-pc platform.
       > machine # /nix/store/6934ywwmhnxzrim4f8hwff6c22v0sf6i-grub-2.12/sbin/grub-install: warning: File system `ext2' doesn't support embedding.
       > machine # /nix/store/6934ywwmhnxzrim4f8hwff6c22v0sf6i-grub-2.12/sbin/grub-install: error: embedding is not possible, but this is required for cross-disk install.
       > machine # /nix/store/j07f0cnyhdx46j9n3a40xhjk7z9d5njc-install-grub.pl: installation of GRUB on /dev/vda failed: No such file or directory
       > machine # Failed to install bootloader

I'm not sure what's wrong there, as the example I added is almost exactly like my real config (minus LUKS and BTRFS subvolumes) and I also tried my real config as an example which doesn't work either.
In my NixOS configuration I have boot.loader.grub.efiSupport = true; set, but that doesn't seem to make a difference here as the error from the example does not go away with it.

@Lassulus @Enzime @iFreilicht @Mic92 @phaer can you help me out with this one?

[provokateurin]

This should be an assertion for system.boot.loader.id == "grub", but I'm not sure how to access config here.