Fix windows ASIO clobbering scheduler index bug introduced in #4646 (#4652)

In #4646, we started using a dedicated scheduler index for the ASIO
thread. This was generally a good thing but on windows due to how
IOCP works, we could accidentally overwrite the scheduler index for
a normal scheduler if it happened to be the one receiving the ASIO
callback.

This commit fixes the bug to ensure that we will not overwrite the
scheduler index during ASIO callbacks on windows.

Author: dipinhora

src/libponyrt/asio/event.c

@@ -134,7 +134,10 @@ PONY_API void pony_asio_event_send(asio_event_t* ev, uint32_t flags,
 #ifdef PLATFORM_IS_WINDOWS
   // On Windows, this can be called from an IOCP callback thread, which may
   // not have a pony_ctx() associated with it yet.
-  ponyint_register_asio_thread();
+  // Don't call `ponyint_register_asio_thread` because that would overwrite the
+  // scheduler index if this is run on a normal scheduler thread and that would
+  // be not good.
+  pony_register_thread();
 #endif
 
   asio_msg_t* m = (asio_msg_t*)pony_alloc_msg(POOL_INDEX(sizeof(asio_msg_t)),