fix(reviewer-bot): make spec-lock updates pure

[PLeVasseur]

Summary

• stop make.py --update-spec-lock-file from continuing into the full docs build path so /accept-no-fls-changes only performs the intended spec-lock update work
• preserve the existing success and failure semantics by exiting with an explicit status immediately after the spec-lock update step
• add a regression test proving update-spec-lock mode exits before build_docs(...) is invoked

Testing

• uv run ruff check --fix builder/build_cli.py scripts/reviewer_bot_lib/automation.py .github/reviewer-bot-tests/test_reviewer_bot.py
• uv run python -m pytest .github/reviewer-bot-tests/test_reviewer_bot.py .github/reviewer-bot-tests/test_main.py

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