src: refactor `Environment::GetCurrent(isolate)` usage

Do not require an explicit `HandleScope`, or the ability to create
one, when using `Environment::GetCurrent()`.

`isolate->InContext()` is used as an indicator that it is probably
okay to create a `HandleScope`, see also the short discussion in
https://github.com/nodejs/node/pull/25775#pullrequestreview-197371049.

PR-URL: https://github.com/nodejs/node/pull/26376


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>