Build the lost rekey msg3 test on Windows, where no decrypt worker pool exists

The test asserted that node 0 decrypts inline by reading the supervisor's
decrypt pool field, which exists only on Unix, so the Windows test build
failed. Windows never spawns the pools and always decrypts inline, so the
assertion is needed only where a pool could exist.

src/node/tests/session.rs

@@ -1873,6 +1873,9 @@ async fn a_lost_link_rekey_msg3_is_still_resent_after_an_old_session_frame() {
         ..
     } = rekey_pair_cut_over_with_held_msg3().await;
     drop(held_msg3);
+    // Windows never spawns the worker pools, so there the decrypt is
+    // always inline and the pool field does not exist.
+    #[cfg(unix)]
     assert!(
         nodes[0].node.supervisor.decrypt_workers.is_none(),
         "node 0 must decrypt inline, so this exercises the inline confirm"