pki: Introduce 'write-acl', and fix wrong conversion in 'ensure-acl'.

* guix/pki.scm (write-acl): New procedure.
  (ensure-acl): Use it.  Fixes a regression introduced in 39831f16,
  whereby 'ensure-acl' would yield a wrong-type-arg error.
* guix/scripts/archive.scm (authorize-key): Use 'write-acl'.