worker,etw: only enable ETW on the main thread

The Windows ETW code is not written to be compatible with multi
threading, and in particular it relies on global state like a
single static `uv_async_t`. Adding that to multiple threads
would corrupt the corresponding loops' handle queues.

This addresses the flakiness of at least
`test-worker-exit-code` and very likely other flaky tests that
relate to Worker threads on Windows as well.

Fixes: #25847
Fixes: #25702
Fixes: #24005
Fixes: #23873

PR-URL: #25907
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/node_dtrace.cc

@@ -288,7 +288,11 @@ void InitDTrace(Environment* env, Local<Object> target) {
   }
 
 #ifdef HAVE_ETW
-  init_etw();
+  // ETW is neither thread-safe nor does it clean up resources on exit,
+  // so we can use it only on the main thread.
+  if (env->is_main_thread()) {
+    init_etw();
+  }
 #endif
 
 #if defined HAVE_DTRACE || defined HAVE_ETW