feat: disable building XO 6 by default and add config option to sample config file

sample.xo-install.cfg

@@ -138,3 +138,8 @@ PRESERVE="3"
 # Configurable network-timeout setting for yarn, in ms.
 # default: 300000 = 300sec
 #YARN_NETWORK_TIMEOUT="300000"
+
+# Enable/Disable XO 6 UI build, accessible at /v6
+# This is actively developed and enabling it might break install/update
+# default: false
+#INCLUDE_V6="false"