formula: update pour_bottle? CLT error message

[carlocab]

• 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?

---

We have received multiple bug reports from users complaining that brew
tells them to install the CLT when they already have Xcode installed.
See, for example, #18976.

Let's try to avoid these issues from being opened by making the error
message more explicit that we require the CLT at a specific location,
and that installing Xcode does not suffice.

[MikeMcQuaid]

This seems like a good idea to clarify, thanks @carlocab!