[Nix] Don't be verbose if build user key already has the right permissions

nix/builder.nix

@@ -164,7 +164,7 @@ in
     system.activationScripts = {
       buildUserKeySetup.text = ''
         cp --force -v ${buildUserKeyFile} ${buildUserKeyFilePath}
-        chmod -v 400 ${buildUserKeyFilePath}
+        chmod -c 400 ${buildUserKeyFilePath}
       '';
     };
   };