use terranix flake parts module directly

terranixConfigurations.nix

@@ -1,22 +1,17 @@
 { inputs, ... }:
 
-let
-  lib = inputs.nixpkgs.lib;
-  utils = import ./utils.nix { inherit inputs lib; };
-  inherit (utils) mkTerranixDerivation;
-
-  configs = {
-    # Example:
-    # myconfig = {
-    #   modules = [ ./terraform/myconfig.nix ];
-    # };
-  };
-in
-
 {
-  perSystem =
-    { system, ... }:
-    {
-      packages = mkTerranixDerivation { inherit system configs; };
+  imports = [
+    inputs.terranix.flakeModule
+  ];
+
+  perSystem = {
+    terranix.terranixConfigurations = {
+      # Example:
+      # myconfig = {
+      #   modules = [ ./terraform/myconfig.nix ];
+      #   extraArgs = { };  # optional
+      # };
     };
+  };
 }