cask/audit: add arch to signing audit skiplist

[bevanjkay]

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This makes an adjustment to our signing audit skiplist for homebrew-cask to make it architecture specific. We have seen an increasing number of casks that fail only on the GitHub intel-based CI runners. Making this change allows us to skip these cases once they have been manually verified.

The alternative considered is to skip this audit entirely on the macos-15-intel runner, but I don't believe the issue is widespread enough for this to be the best option at this time.

Needed for: Homebrew/homebrew-cask#234707

[Copilot]

Pull request overview

This PR modifies the signing audit skiplist logic in homebrew-cask to support architecture-specific exceptions. Currently, signing audit failures can only be skipped globally for a cask. With this change, casks can be skipped for signing audits on specific architectures (e.g., Intel or ARM) while still being audited on others.

Changes:

• Updated the audit_signing method to check for architecture-specific skiplist entries before falling back to architecture-agnostic ("all") entries

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