github_packages: handle "manifest unknown"

[Bo98]

• 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?

---

This seems to be very similar to "name unknown".

[BrewTestBot]

Review period will end on 2021-04-13 at 18:00:06 UTC.

[BrewTestBot]

Review period skipped due to critical label.

[MikeMcQuaid]

Makes sense, thanks @Bo98!