Google K8s Mirror offline?
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 -