Fix signing diag file consolidation: always run

[dagood]

The signing diagnosis artifact was only being created upon success of the main signing process. This works for diagnosing false successes, but not actual errors. Use defer to always create it.

Also, consolidate as many files as possible rather than stopping upon error.

• Spotted while looking at https://github.com/microsoft/go-lab/issues/289

[Copilot]

Pull request overview

This PR fixes the signing diagnostic file consolidation process to ensure it always runs, even when the main signing process fails. Previously, diagnostic artifacts were only created on success, which prevented diagnosing actual errors. The fix uses a deferred function to guarantee execution, and implements error accumulation to consolidate as many files as possible instead of stopping at the first error.

Changes:

• Moved diagnostic file consolidation to a deferred function in run() to ensure it executes regardless of success or failure
• Refactored error handling in consolidateDiagnosticFiles() to accumulate errors using errors.Join rather than returning immediately on first error

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
eng/_util/cmd/sign/sign.go	Adds defer block to ensure diagnostic consolidation always runs and joins consolidation errors with main errors using named return value
eng/_util/cmd/sign/archive.go	Refactors consolidateDiagnosticFiles() to accumulate all errors with errors.Join instead of early returns

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