Reach backend via pinned turbo_tasks instead of cloning self
idle_start and try_read_task_output each took self: &Arc<Self> only to
move a self.clone() into a deferred task/closure. But both already pin
turbo_tasks (Arc<TurboTasks>), which owns the backend, so the spawned
work can reach the backend through turbo_tasks.backend().0 — its lifetime
is anchored by the pin. Drop both clones and take &self.
This removes the last self: &Arc<Self> receivers and self.clone() calls
in the crate; the inner Arc is no longer used for shared ownership.
