src: use persistent strings from node::Environment

Replace a few calls to FIXED_ONE_BYTE_STRING() with their persistent
counterparts from `node::Environment`.  None of the calls are in hot
code paths but why create a new string when one already exists?

PR-URL: https://github.com/nodejs/node/pull/11945


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>