bin/brew: only export CI if not set.

[MikeMcQuaid]

Also: GitHub Actions already sets it.

• Have you followed the guidelines in our Contributing document?
• Have you checrked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

[MikeMcQuaid]

Working: https://github.com/Homebrew/brew/pull/8247/checks?check_run_id=958091005#step:13:77