I have a patch, what do you think?

I tested it by building an operating system of the form:

(operating-system
  ...
  (services
   (append
       (list
        (service openssh-service-type
                 (openssh-configuration
                  (authorized-keys
                   `(("root" ,(plain-file "first-key" "ssh-rsa ..."))
                     ("root" ,(plain-file "second-key" "ssh-rsa ..."))))))))))

I caught the derivation to build the authorized-keys directory, and root
had 2 keys. Without the patch, root had only 1 key.

Vivien