Pin actions to commits

Use specific commit hashes instead of version tags when referencing GitHub Actions in CI/CD workflows. Version tags can be moved or updated, potentially introducing security vulnerabilities or breaking changes, while commit hashes are immutable and provide reproducible builds.

Replace version tags like @v4 or @v3 with the full commit hash:

# Instead of:
- uses: actions/checkout@v4
- uses: DeterminateSystems/determinate-nix-action@v3

# Use:
- uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8
- uses: DeterminateSystems/determinate-nix-action@b7303d63f88908d15f0bcb207e60b3a0ea7f1712

This practice enhances security by preventing supply chain attacks where malicious actors could update the code behind a version tag, and ensures reproducible builds by guaranteeing the exact same action code runs every time.