install: Register the hydra.gnu.org key on the installation image.

* gnu/services/base.scm (hydra-key-authorization): New procedure.
  (guix-service): Add #:authorize-hydra-key? parameter; honor it using
  'hydra-key-authorization'.
* gnu/system/install.scm (installation-services): Pass
  #:authorize-hydra-key? #t.