Pin GitHub Actions to full-length commit SHAs - #477

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-node@3235b876344d2a9aa001b8d1453c930bba69e610. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/