TrigServices: Use ROOT in athenaHLT always in batch mode

Always put ROOT into batch mode to avoid additional GUI threads being
created that could cause problems during forking (ATR-21890).

HLT/Trigger/TrigControl/TrigServices/python/TriggerUnixStandardSetup.py

@@ -68,7 +68,7 @@ def setupCommonServices():
 
     # setup ROOT6
     from PyUtils.Helpers import ROOT6Setup
-    ROOT6Setup()
+    ROOT6Setup(batch=True)
 
     # Setup online THistSvc unless specifically configured otherwise
     #    setup the THistSvc early and force the creation of the THistSvc