An unforeseen consequence of PR #3833 landing was that `tauri dev`
stopped working reliably, getting softlocked when the `app-lib` crate
build script actually needed to build Java scripts: Gradle always
modifies a few files under the `.gradle` directory when run, which get
picked up by Tauri as source code changes that should trigger a rebuild,
but such rebuild triggers Gradle to run and modify those files again ad
infinitum.
This change fixes that by adding such a directory to a documented Tauri
exclusion file, restoring such functionality back.
