Previously one would need to have a worktree for the local 'keyring'
branch.
Reported by reepca and bricewge on #guix.
* build-aux/git-authenticate.scm (load-keyring-from-reference): Use
'branch-lookup' instead of 'reference-lookup'. Add "origin/" to
REFERENCE.
(authenticate-commits): Have #:keyring-reference default to "keyring".