give collectibales physics layer

this keeps keys, dropped-hedz and cash from "colliding" with the camera arm

src/tb_entities.rs

@@ -168,6 +168,7 @@ impl CashSpawn {
             SceneRoot(mesh),
             Cash,
             Collider::cuboid(2., 3.0, 2.),
+            CollisionLayers::new(LayerMask(GameLayer::Collectibles.to_bits()), LayerMask::ALL),
             CollisionEventsEnabled,
             Sensor,
         ));