fix: catch CondaBuildUserError w/ SystemExit

[beckermr]

Description

This PR catches CondaBuildUserError with SystemExit in internal routines in the code. What is going on is that as we've slowly been moving sys.exit calls to raising a CondaBuildUserError, we've inadvertently started raising exceptions in places where we should not be.

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[codspeed-hq]

CodSpeed Performance Report

Merging #5538 will not alter performance

_{Comparing beckermr-patch-1 (5bffa4d) with main (41856b5)}

Summary

✅ 5 untouched benchmarks

[beckermr]

@conda/builds-tools this one is ready for review!