pull: Do not use '~*', which 'msgfmt' fails to interpret.

Fixes <https://bugs.gnu.org/37505>.
Reported by Konrad Hinsen <konrad.hinsen@fastmail.net>.

* guix/scripts/pull.scm (display-channel-news): Use ~a instead of ~*
when reporting new channels.

guix/scripts/pull.scm

@@ -304,7 +304,7 @@ to display."
                (new
                 (let ((count (length new)))
                   (format (current-error-port)
-                          (N_ "  ~*One new channel:~%"
+                          (N_ "  ~a new channel:~%"
                               "  ~a new channels:~%" count)
                           count)
                   (for-each display-channel new))))