detect Bun installs in CLI update banner (#5074)

## Summary
- detect Bun-managed installs in the JavaScript launcher and set a
dedicated environment flag
- show a Bun-specific upgrade command in the update banner when that
flag is present

Fixes #5012

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https://chatgpt.com/codex/tasks/task_i_68e95c439494832c835bdf34b3b1774e

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Co-authored-by: Michael Bolin <mbolin@openai.com>