Pin GitHub Actions to full-length commit SHAs - #359

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@1d96c772d19495a3b5c517cd2bc0cb401ea0529f. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/