release-haskell.nix: temporarily disable x86_64-darwin
release-haskell.nix: temporarily disable x86_64-darwin

rustc fails to build on x86_64-darwin on the M1 builders Hydra uses.
Since GHC depends on rustc (sphinx -> cryptography -> GHC), the entire
darwin package set is broken at the moment, cluttering build reports
etc.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs