Previously, the "ar command" in the global config of GHC in nixpkgs is
simply "ar" instead of a proper absolute path in the nix store. This
will result in an "ar: command not found" error when using GHC and cabal
in a pure nix shell. This commit adds the patch and applies to all
pre-9.0 versions.
See output of ghc --info for "ar command" value.
