substitute: Make download messages more concise.

'guix substitute' now displays 2 lines per download instead of 4.
Suggested by Ricardo Wurmus.

* guix/scripts/substitute.scm (assert-valid-narinfo): Have #:verbose?
default to #f.  Remove leading newline in message.
(process-substitution): Display the URI rather than the store file name.
Display two newlines after the substitution.