pinwheel: Add nixfmt to nix module

hosts/pinwheel/modules/nix/default.nix

@@ -1,6 +1,9 @@
 { pkgs, ... }:
 {
   home-manager.users.alex = {
-    home.packages = [ pkgs.nix-tree ];
+    home.packages = [
+      pkgs.nixfmt-rfc-style
+      pkgs.nix-tree
+    ];
   };
 }