generate-cask-ci-matrix: skip signing on macOS 15 #24312

In homebrew/cask CI, the signing audit intermittently fails on the macos-15 runner with Signature verification failed for <X>: rejected, while macos-26 and xcode-27 accept the same artifacts in the same run. Locally, the artifacts verify as notarized, and a re-run often clears the failure. Each one costs a maintainer a re-run and an investigation.

macos-15 runners are only included when the cask definition specifically requires the variation.

This PR adds signing to the macos-15 job's --except only when another macOS runner downloads the same URL, so the artifact is still signing-audited elsewhere. A URL that only macos-15 downloads keeps its signing check, as does a cask limited to macos-15. Every other audit still runs on macos-15.

Below is a table that shows how many signing audits will be skipped on macos-15 after this change (1,246), and how many will still be explicitly checked (12).

Casks
Count

Get a macos-15 job
1,258

macos-15 shares its URL with another macOS runner
1,246

macos-15 downloads a URL no other runner audits
12

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Claude with Opus 5.5 at Extra High effort, with local review and testing.