chores: change nixpkgs flake input to the IPV6-compatible one

Author: yvan-sraka

flake.lock

@@ -2,7 +2,7 @@
   description = "Supabase Auth Service with Nix modules and steps";
 
   inputs = {
-    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    nixpkgs.url = "https://channels.nixos.org/nixos-unstable/nixexprs.tar.xz";
     flake-utils.url = "github:numtide/flake-utils";
   };