rubocops/uses_from_macos: audit when depends_on :linux

[cho-m]

• 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?
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• Have you successfully run brew tests with your changes locally?

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The uses_from_macos DSL is a bit confusing when a formula is depends_on :linux as the formula isn't using anything on macOS.

We've typically have used depends_on "<builtin-macos>" for these formulae which sounds more accurate.

[MikeMcQuaid]

Good idea.

[cho-m]

There is one potential scenario of adding depends_on :linux to HEAD that this will give a false positive. Though, we may be able to defer handling this if not used in homebrew-core.

[MikeMcQuaid]

Looks good, thanks @cho-m!