Allow Hydra's binary cache to be signed

This requires adding the following to hydra.conf:

  binary_cache_key_name = <key-name>
  binary_cache_private_key_file = <path-to-private-key>

e.g.

  binary_cache_key_name = hydra.nixos.org-1
  binary_cache_private_key_file = /home/hydra/cache-key.sec