Use appropriate chalk instance for stderr output

[dspinellis]

Chalk auto-detects whether the output channel supports colors, but does that for stderr. Consequently, colors appear inconsistently (and wrongly) depending on whether stdout or stderr have been redirected.

Issue: #13097

Description

This change corrects the instance of chalk used, according to the employed output channel (stdout or stderr).

Checklist

• I manually tested the result. I also tried adding tests to confirm my change works in three ways, but none is viable. Specifically, I tried the following:
  • via changes to run-pretties.js (its existing process.stdin.isTTY code is bogus, because chalk calls directly isatty()),
  • via mocking (it doesn't work, because chalk sets the values at the module level), and
  • by modifying the chalk object with Object.defineProperty (it doesn't work, because the configurable is set to false).
• (If the change is user-facing) I’ve added my changes to changelog_unreleased/*/XXXX.md file following changelog_unreleased/TEMPLATE.md.
• I’ve read the contributing guidelines.

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[fisker]

Thanks for working on this!