make-disk-image: add binfmt emulation

[saviosg]

Now it's possible to build images for other architectures with native speed using KVM.
Binfmt emulation is only needed for the bootloader installation step, other steps use native binaries.

To use it, set imageBuilder.enableBinfmt to true, and pass a new nixpkgs instance to the image builder, with the system set to the running machine.

Example, building an aarch64 image on an x86_64 host:

nixpkgs.lib.nixosSystem {
  system = null;
  modules = [
    {
      nixpkgs.buildPlatform = "aarch64-linux";
      nixpkgs.hostPlatform = "aarch64-linux";
    }
  ];
}

...

imageBuilder.enableBinfmt = true;
imageBuilder.pkgs = nixpkgs { system = "x86_64-linux"; };

[iFreilicht]

Very cool! Could you try to add a test for this as well? No need to test all permutations, aarch64 on x86_64 would be sufficient.

[phaer]

Very nice feature, thank you!

Don't want to block, but I think copying the magic numbers and such from nixpkgs and duplicating them in diskos source might not be the best course of action long-term.

Maybe nixpkgs could expose them via a non-visible readOnly option or so instead?