Control vars are now stored in the globalCtx only.

However, it is still allowed to store control var in a local context in special situation

simple-store.go

@@ -54,7 +54,7 @@ func (ctx *SimpleStore) Merge(src ExprContext) {
 func varsCtxToBuilder(sb *strings.Builder, ctx ExprContext, indent int) {
 	sb.WriteString("vars: {\n")
 	first := true
-	for _, name := range ctx.EnumVars(filterPrivName) {
+	for _, name := range ctx.EnumVars(nil) {
 		if first {
 			first = false
 		} else {