restructure: system modules

system/common/base/nix.nix

@@ -0,0 +1,26 @@
+{ inputs, lib, ... }:
+{
+  nix = {
+    settings = {
+      trusted-users = [ "root" "@wheel" ];
+      auto-optimise-store = lib.mkDefault true;
+      experimental-features = [ "nix-command" "flakes" "repl-flake" ];
+      warn-dirty = false;
+      system-features = [ "kvm" "big-parallel" "nixos-test" ];
+      flake-registry = "";
+    };
+    gc = {
+      automatic = true;
+      dates = "weekly";
+      options = "--delete-older-than 2d";
+    };
+
+    # Add each flake input as a registry
+    # To make nix3 commands consistent with the flake
+    registry = lib.mapAttrs (_: value: { flake = value; }) inputs;
+
+    # Add nixpkgs input to NIX_PATH
+    # This lets nix2 commands still use <nixpkgs>
+    nixPath = [ "nixpkgs=${inputs.nixpkgs.outPath}" ];
+  };
+}