use str.format-style placeholder in mail template

Signed-off-by: Nils Philippsen <nils@redhat.com>

distgit_bugzilla_sync/default-config-files/configuration.toml

@@ -30,7 +30,7 @@ tmpl_admin_email = """
 The following errors were encountered while updating bugzilla with information
 from the Package Database.  Please have the problems taken care of:
 
-%s
+{errors}
 """
 
 # User email for invalid setup

distgit_bugzilla_sync/script.py

@@ -689,7 +689,7 @@ def main():
                 env['email_from'],
                 env['notify_emails'],
                 'Errors while syncing bugzilla with the PackageDB',
-                env['tmpl_admin_email'] % ('\n'.join(errors),))
+                env['tmpl_admin_email'].format(errors='\n'.join(errors))
             )
     else:
         with open(env['data_cache'], 'w') as stream: