Use Thread.delay instead of sleepf for consistency

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

src/dune_engine/scheduler.ml

@@ -477,7 +477,7 @@ end = struct
   let wait_win32 () =
     while not (wait_nonblocking_win32 ()) do
       Mutex.unlock mutex;
-      Unix.sleepf 0.001;
+      Thread.delay 0.001;
       Mutex.lock mutex
     done