Build hadrian with ghc 9.6.6 if available.

Author: hamishmack

Diff for: compiler/ghc/default.nix

@@ -259,6 +259,8 @@ let
       compiler-nix-name =
         if builtins.compareVersions ghc-version "9.4.7" < 0
           then "ghc928"
+        else if buildPackages.haskell.compiler ? ghc966
+          then "ghc966"
         else if buildPackages.haskell.compiler ? ghc964
           then "ghc964"
         else "ghc962";