enable Darwin.arm64 to install x86_64 binary
enable Darwin.arm64 to install x86_64 binary

Throwing @thefloweringash under the bus if this doesn't work, but it
sounds like Apple Silicon devices can use the x86_64 binary for now.

Fixes #4058

NixOS/nix,rycee/nix,NixOS/nix,rycee/nix,rycee/nix,NixOS/nix,rycee/nix,rycee/nix,NixOS/nix,NixOS/nix