Lib/Nixos: Add common nix configuration

lib/nixos.nix

@@ -4,6 +4,26 @@
   lib,
   ...
 }: {
+  # Common nix daemon settings shared between NixOS and nix-darwin.
+  # Darwin additionally needs nix.enable = true.
+  mkCommonNixSettings = username: {
+    enable = true;
+    package = pkgs.nixVersions.stable;
+    extraOptions = ''
+      experimental-features = nix-command flakes pipe-operators
+    '';
+    settings.trusted-users = ["root" username];
+    gc.automatic = false;
+    gc.options = "--delete-older-than 5d";
+    settings.auto-optimise-store = true;
+    optimise.automatic = true;
+    registry = lib.mapAttrs' (n: v: lib.nameValuePair n {flake = v;}) inputs;
+    nixPath = [
+      "nixpkgs=${inputs.nixpkgs.outPath}"
+      "home-manager=${inputs.home-manager.outPath}"
+    ];
+  };
+
   mkNixosConfigWithHomeManagerModule = {
     system,
     mylib,