store: Use the daemon's substitute URLs by default.

Partly fixes <http://bugs.gnu.org/20217>.

* guix/store.scm (set-build-options): Change #:substitute-urls to
  default to #f.  Send the 'substitute-urls' pair only if
  SUBSTITUTE-URLS is true.
* guix/scripts/build.scm (set-build-options-from-command-line): Do not
  default to %DEFAULT-SUBSTITUTE-URLS for #:substitute-urls.
* guix/scripts/size.scm (%default-options): Remove 'substitute-urls'.