refactor config with better structure

nixos/system/nix.nix

@@ -0,0 +1,32 @@
+{ ... }:
+{
+  nix.settings = {
+    experimental-features = [
+      "nix-command"
+      "flakes"
+    ];
+    trusted-users = [ "prouk" ];
+
+    substituters = [
+      "https://cache.nixos.org"
+      "https://nix-community.cachix.org"
+    ];
+    trusted-public-keys = [
+      "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY="
+      "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
+    ];
+  };
+
+  nixpkgs.config.allowUnfree = true;
+
+  # Housekeeping
+  nix.gc = {
+    automatic = true;
+    dates = "weekly";
+    options = "--delete-older-than 14d";
+  };
+  nix.optimise.automatic = true;
+
+  # Run unpatched dynamic binaries.
+  programs.nix-ld.enable = true;
+}