This is meant to ease transition for people running an older guix-daemon
invoking a recent 'guix perform-download' with only one argument.
This is a followup to 9b5364a3af.
* guix/scripts/perform-download.scm (perform-download): Make 'output'
optional. Bind 'output*' from DRV's "out" and honor it.
(guix-perform-download): Add clause with one argument.