I'm having some trouble building `netboot` from 'nixos/release.nix':
```
$ nix-build -A netboot 'nixos/release.nix'
...
error: a 'aarch64-linux' is required to build '/nix/store/0vf0k9kib1z431qd20xyyb20piw2ybmg-append-initrd-secrets.drv', but I am a 'x86_64-linux'
```
Happens on master, nixos-18.03 and nixos-18.09. Is this just me, or does anyone else have the same issue?