Add error for old haskell.nix

Author: hamishmack

default.nix

@@ -0,0 +1,28 @@
+builtins.throw ''
+  This version of hackage.nix is not compatible with older versions of haskell.nix.
+  Newer versions of hackage.nix are significantly smaller, and faster to download.
+
+  However newer versions of haskell.nix are lacking high fidelity support for GHC 8.10,
+  as such windows cross compilation is broken, and more may break going forward.
+
+  You have two options:
+  
+  1. Please update to a newer version of haskell.nix, or
+
+  2. If you cannot update haskell.nix it is recommended that you pin your haskell.nix
+     to your current revision or `cb139fa956158397aa398186bb32dd26f7318784` and point
+     hackage.nix to `github:input-output-hk/hackage.nix?ref=for-stackage` as shown below:
+
+      hackageNix = {
+        url = "github:input-output-hk/hackage.nix?ref=for-stackage";
+        flake = false;
+      };
+  
+      haskellNix = {
+        # GHC 8.10.7 cross compilation for windows is broken in newer versions of haskell.nix.
+        # Unpin this once we no longer need GHC 8.10.7.
+        url = "github:input-output-hk/haskell.nix/cb139fa956158397aa398186bb32dd26f7318784";
+        inputs.hackage.follows = "hackageNix";
+      };
+
+''