restructured core modules

modules/system/core/nix-settings.nix

@@ -0,0 +1,53 @@
+{
+  self,
+  inputs,
+  ...
+}: {
+  flake.nixosModules.nix = {
+    pkgs,
+    lib,
+    ...
+  }: {
+    nix = {
+      registry = {
+        voidarc = {
+          from = {
+            id = "voidarc";
+            type = "indirect";
+          };
+          to = {
+            type = "git";
+            url = "https://git.voidarc.co.uk/voidarc/flakes.git";
+          };
+        };
+      };
+      settings = {
+        cores = 6;
+        download-buffer-size = 524288000;
+        experimental-features = [
+          "nix-command"
+          "flakes"
+        ];
+      };
+      nixPath = ["nixpkgs=${inputs.nixpkgs}"];
+      optimise.automatic = true;
+      gc = {
+        automatic = true;
+        dates = "daily";
+        options = "--delete-older-than 5d";
+      };
+    };
+    nixpkgs = {
+      config = {
+        allowUnfree = true;
+        packageOverrides = pkgs: {
+          unstable = import inputs.nixpkgs-unstable {
+            config = {
+              allowUnfree = true;
+            };
+          };
+        };
+      };
+    };
+  };
+}