cargo-tauri: fix test app eval on darwin

pkgs/by-name/ca/cargo-tauri/test-app.nix

@@ -25,7 +25,7 @@ rustPlatform.buildRustPackage rec {
     };
   };
 
-  postPatch = ''
+  postPatch = lib.optionalString stdenv.hostPlatform.isLinux ''
     substituteInPlace $cargoDepsCopy/libappindicator-sys-*/src/lib.rs \
       --replace "libayatana-appindicator3.so.1" "${libayatana-appindicator}/lib/libayatana-appindicator3.so.1"
   '';