src: restrict unloading addons to Worker threads

Unloading native addons from the main thread was an (presumably
unintended) significant breaking change, because addons may
rely on their memory being available after an `Environment` exits.

This patch only restricts this to Worker threads, at least for the
time being, and thus matches the behaviour from
https://github.com/nodejs/node/pull/23319.

PR-URL: https://github.com/nodejs/node/pull/25577
Refs: https://github.com/nodejs/node/pull/24861
Refs: https://github.com/nodejs/node/pull/23319


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>