Use regular checkout instead of github action checkout v5.

[wstcliyu]

Description

Use regular checkout for pathways test workflows instead of github action checkout v5.

Sometimes the github runner pod can crash before the post checkout action. This change will skip the post checkout step (the post checkout step is not needed anyway since we are getting a new environment every run).

FIXES: b/521475816

Tests

The post checkout step is gone: https://github.com/AI-Hypercomputer/maxtext/actions/runs/27307583883/job/80669551875

Checklist

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• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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