http://hydra.nixos.org/build/33279996
The public key can be derived from the secret key, so there's no need for the user to supply it separately.
This is currently only used by the Hydra queue runner rework, but like eff5021eaa it presumably will be useful for the C++ rewrite of nix-push and download-from-binary-cache. (@shlevy)
eff5021eaa