koji_builder / stg: perhaps we can just supress the warning

Signed-off-by: Kevin Fenzi <kevin@scrye.com>

roles/koji_builder/templates/builders/site-defaults.cfg

@@ -7,5 +7,5 @@ config_opts['nosync_force'] = True
 config_opts['environment']['LANG'] = 'C.UTF-8'
 config_opts['use_bootstrap'] = False
 {% if env == 'staging' %}
-config_opts['package_manager'] = 'yum'
+config_opts['dnf_warning'] = False
 {% endif %}