n-api: back up env before finalize

Heed the comment to not use fields of a Reference after calling its
finalize callback, because such a call may destroy the Reference.

Fixes: https://github.com/nodejs/node/issues/19673
PR-URL: https://github.com/nodejs/node/pull/19718


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>