update-report: still report homebrew/core changes if they are updated

[Rylan12]

• 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?

---

Follow-up to #11729

We also need to check whether it's a preinstall update (i.e. that --preinstall was passed). Otherwise, homebrew/core will still be updated and its updates won't be listed in the output because they'll be ignored by HOMEBREW_JSON_CORE presence.

[BrewTestBot]

Review period skipped due to critical label.

[MikeMcQuaid]

👍🏻