make-disk-image: fix build for systems that use boot.loader.grub.devices

[Mic92]

config.boot.loader.grub.device is just an alias that gets assigned to config.boot.loader.grub.devices. If config.boot.loader.grub.device is set to null, it will fail with the following error as described in nix-community/nixos-generators#339

Description of changes

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[Mic92]

$ nixos-generate -f do -I nixpkgs=https://github.com/Mic92/nixpkgs/archive/refs/heads/digital-ocean.tar.gz

[Mic92]

I started to do a regression test over all the images here: nix-community/nixos-generators#341 and here nix-community/nixos-images#239

[Mic92]

• Builds for all images in nixos-generators and nixos-images.

[github-actions]

Successfully created backport PR for release-24.05:

• [Backport release-24.05] make-disk-image: fix build for systems that use boot.loader.grub.devices #322178