Fix building ghc964 with latest nixpkgs-unstable pin

See #2330

Author: hamishmack

ci.nix

@@ -63,6 +63,7 @@
         ghc910 = true;
         ghc912 = true;
       } // nixpkgs.lib.optionalAttrs (nixpkgsName == "unstable") {
+        ghc964 = false;
         ghc96 = false;
         ghc96X = true;
         ghc98 = true;