golang: update CI to use librarian

[JoeWang1127]

We need to create a CI in google-cloud-go to run librarian generate -all, the CI will pass if no changes detected in the repo.

The CI can find:

• unintentional change in generated files
• breaking changes in librarian.yaml

[jskeet]

Could you clarify which CI tasks you expect to use librarian? I'm not expecting us to have scheduled automation performing generation/releasing just yet. (We'll be running commands on Cloudtops.)

[JoeWang1127]

Could you clarify which CI tasks you expect to use librarian?

Added the description.