我有几个 Google Clouds 计算实例（美国、德国、澳大利亚）
在做的时候
apt-get update
今天我得到：
> Get:10 http://packages.cloud.google.com/apt
> google-cloud-packages-archive-keyring-stretch InRelease [3,876 B]
> Err:6 http://packages.cloud.google.com/apt cloud-sdk-stretch InRelease
> The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6A030B21BA07F4FB
> Err:7 http://packages.cloud.google.com/apt google-compute-engine-stretch-stable InRelease
> The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6A030B21BA07F4FB
> Err:10 http://packages.cloud.google.com/apt google-cloud-packages-archive-keyring-stretch InRelease
> The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6A030B21BA07F4FB
在所有这些。有什么我需要做的吗，或者这是 …