4b836fb680 added `pkgs.grub2_efi` to `environment.systemPackages` so that it would be in the Nix store and available for install. But `pkgs.grub2` is already in the list. This causes the various paths of the two GRUB2 versions to collide. To fix this, put `pkgs.grub2_efi` into `system.extraDependencies` instead. This should achieve the same effect of adding the second GRUB2 version to the Nix store without the paths colliding in the environment.
To reproduce the problem, execute `nix-build nixos -I nixos-config=nixos/modules/installer/cd-dvd/iso-image.nix -A config.system.build.isoImage` and look for messages like
```
warning: collision between `/nix/store/9jk1p9n5dl431lcm4w9p6x6x8a00dm0q-grub-2.12/bin/grub-install' and `/nix/store/809l0i6aydg4zhn3kqf723brjyp2qm8h-grub-2.12/bin/grub-install'
```
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