pr-pull: skip bottles for CI-syntax-only PRs

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

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Don't try to download bottle artifacts when merging PRs labeled as CI-syntax-only. This will allow CI-syntax-only PRs to be automerged.

Before, automerging would usually fail because pr-pull tried to download bottle artifacts that CI never created (see Homebrew/homebrew-core#64740 and the attempted merge for an example).

[MikeMcQuaid]

Nice work, thanks @Rylan12!