Add pkgs overlay

modules/nix.nix

@@ -1,4 +1,4 @@
-{ pkgs, ... }:
+{ pkgs, inputs, ... }:
 
 {
   nix = {
@@ -15,6 +15,18 @@
     autoOptimiseStore = true;
   };
 
+  nixpkgs.overlays = with inputs; [
+    self.overlay
+    (final: prev: {
+      unstable = import nixpkgs-unstable {
+        inherit (config.nixpkgs)
+          config
+          overlays
+          system;
+      };
+    })
+  ];
+
   environment.systemPackages = with pkgs; [
     cached-nix-shell
   ];