fix: check out the repo in the publish job so uv finds testpypi

[jamrce]

Changes

• Fix to enable publish job to find testpypi index to publish to pypi through CI/CD

Checklist

• If this changes what a generator writes: an issue was opened first (CONTRIBUTING.md)
• ruff check / ruff format --check / mypy / pytest all pass locally
• Tests added or updated, including a determinism assertion in tests/test_e2e.py if this writes anything a release commits
• CHANGELOG.md's Unreleased section updated
• README command table / COMMANDS registry updated, if a subcommand or its flags changed