Add builds for macOS

[dwhjames]

Include darwin amd64 & arm64 in binary releases.

[dwhjames]

@micromaomao thanks for your attention over in

• Add builds for macOS integrity-vhd#21

I see that you also are recently working on this codebase

• Implement handling of COSE_KeySet and TTL payload #42

My goal here is to get macOS binaries supported in

• https://github.com/Azure/azure-cli-extensions/blob/main/src/confcom/azext_confcom/cose_proxy.py

[Copilot]

Pull request overview

This PR updates the GitHub Actions release workflow to include macOS (darwin) builds in the generated artifacts and draft GitHub release assets, aligning binary releases with darwin amd64 and arm64 support.

Changes:

• Add cross-compiled darwin/amd64 and darwin/arm64 go build steps producing sign1util-darwin-amd64 and sign1util-darwin-arm64.
• Upload the new macOS binaries as part of the existing binaries artifact.
• Attach the new macOS binaries to the draft GitHub release alongside existing Windows and Linux assets.

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