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: https://github.com/nodejs/node/issues/25847
Fixes: https://github.com/nodejs/node/issues/25702
Fixes: https://github.com/nodejs/node/issues/24005
Fixes: https://github.com/nodejs/node/issues/23873

PR-URL: https://github.com/nodejs/node/pull/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>