Take &self in inner run_backend_job instead of &Arc<Self>
The Backend trait already declares run_backend_job with &self. The inner
TurboTasksBackendInner method took self: &'a Arc<Self> only to do
let this = self.clone(), but that clone was used purely synchronously
within the same loop iteration (snapshot_and_persist / should_evict /
evict_after_snapshot, all &self) and never moved into a spawn. The
background future's lifetime is anchored by the engine holding the
TurboTasks Arc, not by the backend cloning its own inner Arc. Drop the
gratuitous clone and take &self.
