Pin GitHub Actions to full-length commit SHAs - #624

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@0717577d45739eb3c851188b29f50ed6c0b2194e, actions/setup-python@e9aba2c848f5ebd159c070c61ea2c4e2b122355e. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/