Fix inverting the user cache

In the previous commit we changed the key from ``bugzilla_email``
to simply ``email`` to be consistent with the data we retrieve
from FAS, but it looks like we forgot to update this method which
thus results in the script failing with a KeyError on
``bugzilla_email``.

Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>

distgit_bugzilla_sync/script.py

@@ -228,7 +228,7 @@ class BugzillaProxy:
         that the bugzilla_email is the key and the username the value.
         """
         for username in self.user_cache:
-            bz_email = self.user_cache[username]["bugzilla_email"].lower()
+            bz_email = self.user_cache[username]["email"].lower()
             self.inverted_user_cache[bz_email] = username
 
     def _get_bugzilla_email(self, username):