libexpr: move ExprLet::attrs data to arena as well

I missed this because I assumed all Exprs were recursed into by
bindVars, but ExprLet's ExprAttrs field is not really its own AST node,
so it doesn't get recursed into.

Author: Radvendii

src/libexpr/nixexpr.cc

@@ -504,6 +504,7 @@ void ExprCall::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
 
 void ExprLet::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
+    attrs->moveDataToAllocator(es.mem.exprs.alloc);
     auto newEnv = [&]() -> std::shared_ptr<const StaticEnv> {
         auto newEnv = std::make_shared<StaticEnv>(nullptr, env, attrs->attrs->size());