fix(nix): make style app check-only

src/dotfiles/apps.nix

@@ -1,7 +1,7 @@
 {
   inputs,
   lib,
-  formatter,
+  formattingCheck,
   system,
 }:
 let
@@ -13,12 +13,11 @@ in
     program = lib.getExe (pkgs.writeShellApplication {
       name = "style-check";
       runtimeInputs = [
-        formatter
         pkgs.statix
         pkgs.deadnix
       ];
       text = ''
-        treefmt --fail-on-change
+        test -e ${formattingCheck}
         statix check .
         deadnix --fail --no-lambda-pattern-names --no-underscore .
       '';

src/dotfiles/default.nix

@@ -53,7 +53,7 @@ in
       lib
       system
       ;
-    formatter = formatter.config.build.wrapper;
+    formattingCheck = formatter.config.build.check self;
   };
 
   nixosConfigurations = import ./hosts.nix {