Google K8s Mirror offline? Heys dudes,
More someone got error when trying update cache of the Google K8s repositories?
I have been got the error about NO_PUBKEY
Err:10 https://packages.cloud.google.com/apt kubernetes-xenial InRelease        
  The following signatures were invalid: EXPKEYSIG 6A030B21BA07F4FB Google Cloud Packages Automatic Signing Key &lt;gc-team@google.com&gt; The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8B57C5C2836F4BEB

and
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://packages.cloud.google.com/apt kubernetes-xenial InRelease: The following signatures were invalid: EXPKEYSIG 6A030B21BA07F4FB Google Cloud Packages Automatic Signing Key &lt;gc-team@google.com&gt; The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8B57C5C2836F4BEB
W: Failed to fetch http://apt.kubernetes.io/dists/kubernetes-xenial/InRelease  The following signatures were invalid: EXPKEYSIG 6A030B21BA07F4FB Google Cloud Packages Automatic Signing Key &lt;gc-team@google.com&gt; The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8B57C5C2836F4BEB
W: Some index files failed to download. They have been ignored, or old ones used instead.

Something was changed?
As per compute/docs/troubleshooting/known-issues:

On Debian and Ubuntu based systems, including your local workstations,
you might encounter an error similar to the following example:
[...] The following signatures were invalid: XXXX [...]

To resolve this error, get the latest valid apt-key.gpg key file from https://packages.cloud.google.com by running the following command:
curl https://packages.cloud.google.com/apt/doc/apt-key.gpg | sudo apt-key add -