Disable testing against nixUnstable on macOS
Disable testing against nixUnstable on macOS

This is failing randomly at the moment which isn't very helpful.

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