formula: update pour_bottle? CLT error message

[carlocab]

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• Have you added an explanation of what your changes do and why you'd like us to include them?
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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.