Motivation for this change

The most user-visible change is that this adds support for nixos-rebuild build-vm-with-bootloader on aarch64-linux.

Take a look at these commits in order so they make more sense. They were unfortunately not independently separable into multiple PRs, as the later commits depend on the earlier ones.

These changes also help set the foundation for testing secure boot with systemd-boot properly: #53901 (comment)

I've done the following to test, both on x86_64-linux and aarch64-linux:

$ nix-build ./nixos/tests/systemd-boot.nix

as well as:

$ nix-build ./nixos -A vmWithBootLoader -I nixos-config=./test.nix
$ ./result/bin/run-nixos-vm -nographic -serial stdio -monitor none

where test.nix is:

{
  boot.loader.systemd-boot.enable = true;
  boot.loader.efi.canTouchEfiVariables = true;
  virtualisation.useEFIBoot = true;
}

CC @samueldr @lheckemann @flokli

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.