cask/audit: disallow new cask to have token in tap_migrations.json

[SeekingMeaning]

• 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?
• Have you successfully run brew man locally and committed any changes?

---

This audit already exists for formulae (but not for casks): https://github.com/Homebrew/brew/blob/master/Library/Homebrew/formula_auditor.rb#L688-L699

Related: Homebrew/homebrew-cask#98719

[BrewTestBot]

Review period will end on 2021-01-27 at 09:20:43 UTC.

[BrewTestBot]

Review period ended.

[MikeMcQuaid]

Thanks @SeekingMeaning!