nixpkgs: init config as separate file

common/default.nix

@@ -5,6 +5,7 @@
     ./environment.nix
     ./home-manager.nix
     ./nginx.nix
+    ./nixpkgs.nix
     ./openssh.nix
     ./security.nix
     ./zsh.nix

common/nixpkgs.nix

@@ -0,0 +1,14 @@
+{ config, lib, pkgs, inputs, ... }:
+
+{
+  nixpkgs = {
+    config = {
+      allowUnfree = true;
+      packageOverrides = pkgs: {
+        agenix = inputs.agenix.packages.${config.nixpkgs.hostPlatform.system}.default;
+      };
+    };
+
+    overlays = [ inputs.nur.overlay ];
+  };
+}