nix: keep old hlint

This keeps the pre-nixpkgs update hlint version, so we can defer
addressing the warnings to a follow-up PR.

default.nix

@@ -141,7 +141,23 @@ rec {
 
   # Linting and styling tools.
   style =
-    pkgs.callPackage nix/tools/style.nix { inherit hsie; };
+    let
+      # keep old version of hlint to not deal with linter changes for now
+      nixpkgsVersionPrev = {
+        date = "2021-11-02";
+        rev = "7053541084bf5ce2921ef307e5585d39d7ba8b3f";
+        tarballHash = "1flhh5d4zy43x6060hvzjb5hi5cmc51ivc0nwmija9n8d35kcc4x";
+      };
+
+      nixpkgsPrev =
+        builtins.fetchTarball {
+          url = "https://github.com/nixos/nixpkgs/archive/${nixpkgsVersionPrev.rev}.tar.gz";
+          sha256 = nixpkgsVersionPrev.tarballHash;
+        };
+      pkgsPrev = import nixpkgsPrev { };
+      inherit (pkgsPrev) hlint;
+    in
+    pkgs.callPackage nix/tools/style.nix { inherit hlint hsie; };
 
   # Scripts for running tests.
   tests =