maintainers/scripts/test-configurations.nix: skip binary GHCs
maintainers/scripts/test-configurations.nix: skip binary GHCs

While being able to test them is neat (on x86_64-linux they work very
well, actually), we usually don't want to do this, since the set is
only (recommended to be) used to bootstrap GHC. Consequently there is
almost no binary cache and testing them mostly leads to unenlightening
and seemingly endless compilation.

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