Reported by Ludovic Courtès <ludo@gnu.org> on #guix.
This fixes a regression introduced in commit 7171d824.
'insert-text-button' procedure does not allow to override a button face,
so a specialized button type should be used instead.
* emacs/guix-ui-package.el (guix-package-heading): New button type.
(guix-package-info-insert-heading): Use it.