src: make copies of startup environment variables

Mutations of the environment can invalidate pointers to environment
variables, so make `secure_getenv()` copy them out instead of returning
pointers.

PR-URL: https://github.com/nodejs/node/pull/11051


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>