internal/exec/stages/disks: guard against nil tang Thumbprint deref - #2290

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/setup-go@v5, actions/upload-artifact@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Restore cache failed: Dependencies file is not found in /home/runner/work/ignition/ignition. Supported file pattern: go.sum