Bug: Lua can generate wrong code when _ENV is <const>

lparser.c

@@ -468,6 +468,7 @@ static void singlevar (LexState *ls, expdesc *var) {
     expdesc key;
     singlevaraux(fs, ls->envn, var, 1);  /* get environment variable */
     lua_assert(var->k != VVOID);  /* this one must exist */
+    luaK_exp2anyregup(fs, var);  /* but could be a constant */
     codestring(&key, varname);  /* key is variable name */
     luaK_indexed(fs, var, &key);  /* env[varname] */
   }