This allows users to type:
guix archive -f docker emacs
as was already the case for the 'nar' format.
Reported by David Thompson.
* guix/scripts/archive.scm (%default-options): Add 'format'.
(export-from-store): Dispatch based on the 'format' key in OPTS.
(guix-archive): Call 'export-from-store' in all cases when the 'export'
key is in OPTS.