Fixes #45908

What was tested

The legacy boot using qemu-system-i386 -m 1024 -cdrom result/iso/*.iso, and also using the x86_64 variant of qemu.

What was not tested

EFI; while the file is at the right location, I have no 32 bit efi, and copying the 32 bit efi file at the bootx64.efi location won't make this work. I do not know how to boot a 32 bit EFI with QEMU, and initial searches weren't good in finding options.

The risks of having a 32-bit only EFI boot is low enough, though, but it would be better to have it be tested.

Things done

• ✔️ Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • ✔️ NixOS
  • 🆖 macOS
  • ⬜ other Linux distributions
• ⬜ Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• ⬜ Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• ✔️ Tested execution of all binary files (usually in ./result/bin/) → the iso itself
• ⬜ Determined the impact on package closure size (by running nix path-info -S before and after)
• ✔️ Fits CONTRIBUTING.md.

cc @vcunat