fix(app-lib): do not softlock tauri dev when a Gradle build is invoked (#3847)

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.

apps/app/.gitignore

@@ -1,6 +1,2 @@
-# Generated by Cargo
-# will have compiled files and executables
-/target/
-
 # Generated by tauri, metadata generated at compile time
 /gen/