src: make watchdog async callback a lambda

`Watchdog::Async` features only once for the
async callback, so make it a lambda.

PR-URL: https://github.com/nodejs/node/pull/25945


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>