fix(treefmt): respect the packages edition

Otherwise it might upgrade to edition 2024 which is not available in
stable.

flake-modules/default.nix

@@ -1,4 +1,8 @@
-{ inputs, ... }:
+{ lib, inputs, ... }:
+let
+  webnsupdate = ../module.nix;
+  cargoToml = lib.importTOML ../Cargo.toml;
+in
 {
   imports = [
     inputs.treefmt-nix.flakeModule
@@ -6,11 +10,7 @@
     ./tests.nix
   ];
 
-  flake.nixosModules =
-    let
-      webnsupdate = ../module.nix;
-    in
-    {
+  flake.nixosModules = {
     default = webnsupdate;
     inherit webnsupdate;
   };
@@ -23,7 +23,10 @@
         projectRootFile = "flake.nix";
         programs = {
           nixfmt.enable = true;
-          rustfmt.enable = true;
+          rustfmt = {
+            enable = true;
+            inherit (cargoToml.package) edition; # respect the package's edition
+          };
           statix.enable = true;
           typos.enable = true;
         };