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.

gc.c

@@ -7228,7 +7228,9 @@ gc_mark_imemo(rb_objspace_t *objspace, VALUE obj)
                 GC_ASSERT(env->ep[VM_ENV_DATA_INDEX_ENV] == obj);
                 GC_ASSERT(VM_ENV_ESCAPED_P(env->ep));
                 rb_gc_mark_values((long)env->env_size, env->env);
-                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);
+                }
                 gc_mark(objspace, (VALUE)rb_vm_env_prev_env(env));
                 gc_mark(objspace, (VALUE)env->iseq);
             }