modularize system config

modules/system/nix-conf.nix

@@ -0,0 +1,15 @@
+{ ... }: {
+  nixpkgs.config.allowUnfree = true;
+  nix = {
+    settings = {
+      experimental-features = [ "nix-command" "flakes" ];
+      auto-optimise-store = true;
+    };
+    gc = {
+      automatic = true;
+      dates = "weekly";
+      options = "--delete-older-than 7d -d";
+    };
+    optimise.automatic = true;
+  };
+}