feat: add unstable nixpkgs overlay

overlays/default.nix

@@ -0,0 +1,11 @@
+# This file defines overlays
+{inputs, ...}: {
+  # When applied, the unstable nixpkgs set (declared in the flake inputs) will
+  # be accessible through 'pkgs.unstable'
+  unstable-packages = final: _prev: {
+    unstable = import inputs.nixpkgs-unstable {
+      system = final.system;
+      config.allowUnfree = true;
+    };
+  };
+}