Fix local-exported vars with "--no-scope-shadowing"

This used to create copies even then, which meant it couldn't modify them.

src/env.cpp

@@ -182,6 +182,7 @@ void var_stack_t::push(bool new_scope) {
 
     // Copy local-exported variables
     auto top_node = top.get();
+    if (new_scope) {
         if (!(top_node == this->global_env)) {
             for (auto& var : top_node->env) {
                 if (var.second.exportv) {
@@ -190,6 +191,7 @@ void var_stack_t::push(bool new_scope) {
                 }
             }
         }
+    }
 
     node->next = std::move(this->top);
     this->top = std::move(node);