fix C++20 compliance with application of upstream PR 965

These symbols are no longer valid on macOS and will fail builds if the `NDEBUG` symbol is defined, as it is now in default build settings for all react-native projects since release of react-native 0.76

This same change was applied upstream but never released, and it appears it has not been applied here

google#965 (comment)

util/env_posix.cc

@@ -837,7 +837,7 @@ class SingletonEnv {
  public:
   SingletonEnv() {
 #if !defined(NDEBUG)
-    env_initialized_.store(true, std::memory_order::memory_order_relaxed);
+    env_initialized_.store(true, std::memory_order_relaxed);
 #endif  // !defined(NDEBUG)
     static_assert(sizeof(env_storage_) >= sizeof(EnvType),
                   "env_storage_ will not fit the Env");
@@ -854,7 +854,7 @@ class SingletonEnv {
 
   static void AssertEnvNotInitialized() {
 #if !defined(NDEBUG)
-    assert(!env_initialized_.load(std::memory_order::memory_order_relaxed));
+    assert(!env_initialized_.load(std::memory_order_relaxed));
 #endif  // !defined(NDEBUG)
   }