src: remove superfluous env_string string

It's only used once at startup in a single place so create the string
in place instead of caching it for the lifetime of the isolate.

PR-URL: https://github.com/nodejs/node/pull/9213


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