Bundlebuilder: relabel repository url as bugtracker

Many activities already have the repository url pointing to their GitHub
page - which also serves as a bug tracker for many of the projects.  The
repository url is not part of the spec, but the bug tracker url is.

src/sugar3/activity/bundlebuilder.py

@@ -461,7 +461,7 @@ class Installer(Packager):
             ET.SubElement(root, 'url', type='homepage').text = \
                 info.get('Activity', 'url')
         if info.has_option('Activity', 'repository_url'):
-            ET.SubElement(root, 'url', type='repository').text = \
+            ET.SubElement(root, 'url', type='bugtracker').text = \
                 info.get('Activity', 'repository_url')
 
         path = os.path.join(prefix, 'share', 'metainfo',