add a missing forward

Author: SteveBronder

stan/math/prim/functor/apply_scalar_binary.hpp

@@ -335,8 +335,8 @@ template <typename F, typename T1, typename T2,
           require_stan_scalar_t<T1>* = nullptr,
           require_std_vector_vt<is_stan_scalar, T2>* = nullptr>
 inline auto apply_scalar_binary(F&& f, T1&& x, T2&& y) {
-  decltype(auto) y_vec = as_column_vector_or_scalar(y);
   using T_return = std::decay_t<decltype(f(x, y[0]))>;
+  decltype(auto) y_vec = as_column_vector_or_scalar(std::forward<T2>(y));
   std::vector<T_return> result(y_vec.size());
   Eigen::Map<Eigen::Matrix<T_return, -1, 1>>(result.data(), result.size())
       = y_vec.unaryExpr(