Fix nix dev environment on aarch64-darwin.

Author: Jan Vogt

flake.lock

@@ -2,7 +2,9 @@
   description = "haskell-language-server development flake";
 
   inputs = {
-    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    # Don't use nixpkgs-unstable as aarch64-darwin is currently broken there.
+    # Check again, when https://github.com/NixOS/nixpkgs/pull/414242 is resolved.
+    nixpkgs.url = "github:NixOS/nixpkgs/c742ae7908a82c9bf23ce27bfca92a00e9bcd541";
     flake-utils.url = "github:numtide/flake-utils";
     # For default.nix
     flake-compat = {