lang: ast: Clear env on ExprCall.Graph

When calling a function, the function is either a parameter, a top-level
variable, a builtin, or a func. None of those should have access to the
enclosing function parameters (not the call site's arguments) which are
in scope at the call site.

lang/ast/structs.go

@@ -8182,7 +8182,7 @@ func (obj *ExprCall) Graph(env map[string]interfaces.Func) (*pgraph.Graph, inter
 	}
 
 	// Add the vertex which produces the FuncValue.
-	exprGraph, funcValueFunc, err := obj.expr.Graph(env)
+	exprGraph, funcValueFunc, err := obj.expr.Graph(map[string]interfaces.Func{})
 	if err != nil {
 		return nil, nil, errwrap.Wrapf(err, "could not get the graph for the expr pointer")
 	}