Add missing write barrier to env_copy()

Previously, the following crashed with
`vm_assert_env:imemo_type_p(obj, imemo_env)` due to missing a missing
WB:

    o = Object.new
    def o.foo(n)
      freeze

      GC.stress = 1

      # inflate block nesting get an imemo_env for each level
      n.tap do |i|
        i.tap do |local|
          return Ractor.make_shareable(-> do
            local + i + n
          end)
        end
      end
    ensure
      GC.stress = false
      GC.verify_internal_consistency
    end

    p o.foo(1)[]

By the time the recursive env_copy() call returns, `copied_env` could
have aged or have turned greyed, so we need a WB for the
`ep[VM_ENV_DATA_INDEX_SPECVAL]` assignment which adds an edge.

Fix: 674eb7d

vm.c

@@ -1225,7 +1225,6 @@ env_copy(const VALUE *src_ep, VALUE read_only_variables)
     }
 
     const rb_env_t *copied_env = vm_env_new(ep, env_body, src_env->env_size, src_env->iseq);
-    volatile VALUE prev_env = Qnil;
 
     if (read_only_variables) {
         for (int i=RARRAY_LENINT(read_only_variables)-1; i>=0; i--) {
@@ -1255,15 +1254,14 @@ env_copy(const VALUE *src_ep, VALUE read_only_variables)
     if (!VM_ENV_LOCAL_P(src_ep)) {
         const VALUE *prev_ep = VM_ENV_PREV_EP(src_env->ep);
         const rb_env_t *new_prev_env = env_copy(prev_ep, read_only_variables);
-        prev_env = (VALUE)new_prev_env;
         ep[VM_ENV_DATA_INDEX_SPECVAL] = VM_GUARDED_PREV_EP(new_prev_env->ep);
+        RB_OBJ_WRITTEN(copied_env, Qundef, new_prev_env);
         VM_ENV_FLAGS_UNSET(ep, VM_ENV_FLAG_LOCAL);
     }
     else {
         ep[VM_ENV_DATA_INDEX_SPECVAL] = VM_BLOCK_HANDLER_NONE;
     }
 
-    RB_GC_GUARD(prev_env);
     return copied_env;
 }