automatic: Use add_security_filters, not _update_security_filters

Resolves https://issues.redhat.com/browse/RHEL-21874

It seems that these two approaches for selecting security updates
sometimes disagree. The regular `dnf update` command uses
base.add_security_filters to select security updates, so dnf-automatic
should do the same.

dnf/automatic/main.py

@@ -380,8 +380,7 @@ def main(args):
 
 def upgrade(base, upgrade_type):
     if upgrade_type == 'security':
-        base._update_security_filters.append(base.sack.query().upgrades().filterm(
-            advisory_type='security'))
+        base.add_security_filters("gte", ("security",))
         base.upgrade_all()
     elif upgrade_type == 'default':
         base.upgrade_all()