By default, once you enter x86_64 Rosetta 2, macOS will try to run
everything in x86_64. So an x86_64 Nix will still try to use x86_64
even when system = aarch64-darwin. To avoid this we can set
kern.curproc_arch_affinity sysctl. With kern.curproc_arch_affinity=0,
we ignore this preference.

This is based on how
https://opensource.apple.com/source/system_cmds/system_cmds-880.40.5/arch.tproj/arch.c.auto.html
works. Completely undocumented, but seems to work!

Note, you can verify this works with this impure Nix expression:

```
  {
    a = derivation {
      name = "a";
      system = "aarch64-darwin";
      builder = "/bin/sh";
      args = [ "-e" (builtins.toFile "builder" ''
        [ "$(/usr/bin/arch)" = arm64 ]
        [ "$(/usr/bin/arch -arch x86_64 /bin/sh -c /usr/bin/arch)" = i386 ]
        [ "$(/usr/bin/arch -arch arm64 /bin/sh -c /usr/bin/arch)" = arm64 ]
        /usr/bin/touch $out
      '') ];
    };

    b = derivation {
      name = "b";
      system = "x86_64-darwin";
      builder = "/bin/sh";
      args = [ "-e" (builtins.toFile "builder" ''
        [ "$(/usr/bin/arch)" = i386 ]
        [ "$(/usr/bin/arch -arch x86_64 /bin/sh -c /usr/bin/arch)" = i386 ]
        [ "$(/usr/bin/arch -arch arm64 /bin/sh -c /usr/bin/arch)" = arm64 ]
        /usr/bin/touch $out
      '') ];
    };
  }
```