Use "/" for wasm's cwd when environment is not avaiable.

src/file.cpp

@@ -54,7 +54,7 @@ inline static std::string wstring_to_string(const std::wstring &wstr)
 inline static std::string get_cwd_from_env()
 {
   char* value = getenv("PWD");
-  if (!value) return "(unreachable)";
+  if (!value) return "/";
   return value;
 }
 #endif