Unbreak lang team shenanigans on Clippy 1.82.

I'm not at all pleased that the lang team thinks it's okay to add a
deny-by-default lint in Clippy where following its instructions breaks
compatibility with, not my older MSRV, but _the previous compiler
version._ Why the compiler can't accept the old 'match x {}' uninhabited
type test, I don't know.

This appears to be the right fix for now.

Upstream bug: rust-lang/rust#129031

os/src/exec.rs

@@ -277,6 +277,7 @@ fn poll_task(
 ) {
     match future.poll(&mut Context::from_waker(&waker_for_task(index))) {
         Poll::Pending => (),
+        #[allow(unreachable_patterns)]
         Poll::Ready(never) => match never {}
     }
 }