ci: pin GitHub Actions to commit SHAs - #55

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Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@a37ce9120846195fa4ece8f58b268e6043cb2f26, actions/setup-python@7f4fc3e22c37d6ff65e88745f38bd3157c663f7c. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/