• Have you followed the guidelines in our Contributing document?
• Have you checked 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 typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

Fixes:

$ eval `brew shellenv`
PATH_HELPER_ROOT=/opt/homebrew: Command not found.

While we're here, also apply the same fix for fallback shells to cover potential future issues.

Fixes #20724.