update-report: don't try to report Homebrew/core if not needed

[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?
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Follow-up to #11715

Auto updates with HOMEBREW_DEVELOPER set will throw an error because HOMEBREW_UPDATE_BEFORE_HOMEBREW_CORE and HOMEBREW_UPDATE_AFTER_HOMEBREW_CORE are unset. This error is ignored unless HMEBREW_DEVELOPER is set so I didn't notice it during my testing.

[BrewTestBot]

Review period skipped due to critical label.

[BrewTestBot]

Review period skipped due to critical label.