Route update-report to Rust

[MikeMcQuaid]

• Keep brew update on update.sh to preserve the update flow.
• Only re-enter brew for update-report when the Rust frontend is set.
• Leave the existing Ruby report call unchanged for normal update runs.

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OpenAI Codex with manual review and tweaks.

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[Copilot]

Pull request overview

This PR aims to let the experimental Rust frontend handle the update-report phase of brew update while keeping the main update flow in the existing shell script. In the broader codebase, this adjusts the frontend-routing logic for one internal command without changing the overall brew update entrypoint.

Changes:

• Wrap the update-report call in cmd/update.sh with a Rust-frontend environment check.
• Add update-report to the Rust frontend allowlist in brew.sh.

Reviewed changes

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

File	Description
Library/Homebrew/cmd/update.sh	Changes how the update flow invokes update-report when the experimental Rust frontend flag is set.
Library/Homebrew/brew.sh	Expands Rust frontend command routing to include the hidden update-report command.

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