fix: write to stderr in utils.env

This is a quick fix to avoid polluting stdout unexpectedly when Poetry's
environment management comes into play.

It's apparent from how much the complexity of this file has grown that
this needs to be refactored moderately, as well as each major class
deserving its own source file.

Future work should also include a rethink of how IO objects are passed
around the codebase, how we reason about verbosity at a function level,
and how code is re-used -- one command may wish to output to stdout, but
if that code is reused by another command, the calculus of what is
command output and what is informative (or even needs to be hidden/shown
based on verbosity level) changes.

Work on output would likely have to be fairly comprehensive and
invasive, but things have grown complex enough that a top-down design
pass is likely the best route.

Regardless, this is a simple change today, and low risk. Resolves #6427.