Use FormulaInstaller OS extensions

[dduugg]

• 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?

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(Vaguely) relates to: #17998

It looks like these OS extensions, introduced in #14163, were not required from formula_installer, so are not actually in use. I fixed that issue, and added some tests intended to prevent a regression. (I don't think I've added OS-specific tests before, so I'm not sure how the magic works, or what assumptions can be made within them, so apologies if this is a bumpy ride.)

[apainintheneck]

😅 Thanks for catching this! I'm glad it only seems like it affected messaging and not formula installs themselves.

[MikeMcQuaid]

Thanks @dduugg!