Configure all NixOS machines to use the internal harmonia binary cache:
- Add nix-cache.johnogle.info as substituter
- Add harmonia signing public key to trusted-public-keys
- Enable fallback for local builds when cache unreachable
- Set 5s connect-timeout for faster fallback
Refs: x-qdkuu
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>