Tell mypy what the closure already knows about placer and pool

Both calls sit in a closure where a narrowing of `Placer | None` and
`PythonWorkerPool | None` will not carry across the function boundary. The
`remote.run_on` line below them already carried the same ignore; these two
close out `make lint-backend`.

backend/fluksio/flow/controller.py

@@ -916,12 +916,12 @@ class FlowController:
         placer, pool = self.placer, self.workers
 
         def call(**kwargs: Any) -> Any:
-            with placer.claim(
+            with placer.claim(  # type: ignore[union-attr]
                 wanted, device=device, policy=policy, node=node_id, run=run_id
             ) as (target, allocation):
                 env = derive_env(wanted, allocation)
                 if target.worker is None:
-                    return pool.for_env(env).run(
+                    return pool.for_env(env).run(  # type: ignore[union-attr]
                         owner,
                         local,
                         code,