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

Author: addaleax

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