shared_audits: add xit to GITHUB_PRERELEASE_ALLOWLIST

[miccal]

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

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Required for Homebrew/homebrew-cask#95593 to pass audit.

[BrewTestBot]

Review period will end on 2020-12-21 at 08:00:52 UTC.

[BrewTestBot]

Review period ended.

[MikeMcQuaid]

@Rylan12 Is this one you expect to be ported over at some stage?

[Rylan12]

Yes, thanks for reminding me. Just re-added this to my to-do list. Depending on when I have availability it might be this weekend. I'd say merge this because so it's not blocking anything. No need to wait for the migration.

[reitermarkus]

@miccal, can you rebase this?