src: cache and resue isolate and contex pointers

Many places these values are obtained through `env` pointer that
incurs function calls. Source in once and re-use as much as possible.
There are more of this pattern in this file, but those are either
one-time use or used in conditional blocks which means sourcing-in
those data early may not be beneficial.

PR-URL: #23024
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>