artwork: Clone over HTTPS.

Suggested by Leo Famulari <leo@famulari.name>
at <https://lists.gnu.org/archive/html/help-guix/2018-11/msg00039.html>.

* gnu/artwork.scm (%artwork-repository): Switch to HTTPS URL.

gnu/artwork.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2014, 2015 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2014, 2015, 2018 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2017 Leo Famulari <leo@famulari.name>
 ;;;
 ;;; This file is part of GNU Guix.
@@ -33,7 +33,7 @@
     (origin
       (method git-fetch)
       (uri (git-reference
-             (url "git://git.savannah.gnu.org/guix/guix-artwork.git")
+             (url "https://git.savannah.gnu.org/git/guix/guix-artwork.git")
              (commit commit)))
       (file-name (string-append "guix-artwork-" (string-take commit 7)
                                 "-checkout"))