inspector: properly shut down uv_async_t

Closing in the Agent destructor is too late, because that happens
when the Environment is destroyed, not when libuv handles are closed.

This fixes a situation in which the same libuv loop is re-used for
multiple Environment instances sequentially, e.g. in our cctest.

PR-URL: https://github.com/nodejs/node/pull/30612


Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>