🤖 ci: keep stderr out of the cold-start JSON artifact

[ThomasK33]

Summary

The desktop cold-start job from #5997 uploads a cold-start.json that is not valid JSON. This PR moves the stdout redirect inside sh -c, so only the runner's JSON reaches the file. Refs #5971. No product change.

Background

The first A/A dispatch (run https://github.com/coder/xum/actions/runs/38013815349, base = head = d0a00215b0, 20 pairs) succeeded, but its artifact starts with 46 lines that are not JSON: 44 runner progress lines and 2 Downloading Electron binary... lines, followed by the JSON document. Ubuntu 22.04's xvfb-run runs its command with "$@" 2>&1 (line 184 of debian/local/xvfb-run on ubuntu/jammy-updates), so the runner's stderr went into the redirected stdout. Current Debian removed the 2>&1, which is why this was easy to miss.

Implementation

xvfb-run -a sh -c 'node ... --json > cold-start.json': the inner shell redirects only the runner's own stdout, so stderr (progress lines) now goes to the job log. The variables still reach the step only through env: and are validated by the existing regex step. A scoped # shellcheck disable=SC2016 marks the single quotes as intended.

Validation

• I ran the exact step text under a stand-in xvfb-run that does "$@" 2>&1, with a stub node. The old form gives a file that does not parse. The new form gives a file that jq -e . parses whole, and the stub's stderr still shows on the console.
• make lint-actions (actionlint and zizmor) and make static-check pass.
• After merge I will dispatch the A/A again and check that the whole uploaded file parses.

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Generated with xum • Model: anthropic:claude-opus-5-5 • Thinking: high

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-10T01:48:33.437785Z	bc063e3	PR opened
🔒 Security Review	✅ Completed 2026-10-10T01:50:04.634837Z	bc063e3	PR opened

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