Avoid unnecessary writes to imemo_env during GC

Similar to the previous commit, to avoid unnecessary Copy-on-Write
memory use we should only set this flag when it has not previously been
set.

imemo.c

@@ -357,7 +357,9 @@ rb_imemo_mark_and_move(VALUE obj, bool reference_updating)
                 ((VALUE *)env->ep)[VM_ENV_DATA_INDEX_ENV] = rb_gc_location(env->ep[VM_ENV_DATA_INDEX_ENV]);
             }
             else {
-                VM_ENV_FLAGS_SET(env->ep, VM_ENV_FLAG_WB_REQUIRED);
+                if (!VM_ENV_FLAGS(env->ep, VM_ENV_FLAG_WB_REQUIRED)) {
+                    VM_ENV_FLAGS_SET(env->ep, VM_ENV_FLAG_WB_REQUIRED);
+                }
                 rb_gc_mark_movable( (VALUE)rb_vm_env_prev_env(env));
             }
         }