[chore] Avoid running poetry at all for env detection in a repl

[blast-hardcheese]

Why

Terminate early if we're running inside a repl.

This will suppress the following poetry commands from showing up in the Packager pane.

What changed

Switch env detection based on whether $REPL_HOME is defined

Test plan

Clear the poetry.lock in order to re-trigger a lock on Run, verify that the poetry env list --full-path command does not show up in the packager pane every time on Run, as it alarms users.