Confirm the Google Cloud account/credentials to test on GitHub Actions with the real Google Cloud Storage

`embulk-input-gcs` is tested on GitHub Actions with the real Google Cloud Storage. For the time being, the test Google Cloud account seems to be maintained by Treasure Data, but there is no guarantee that this account will continue for long.

It shouldn't be a blocker of releasing this plugin. If the test fails by the account failure, in the future, the failure should be just ignored.

Also, the Embulk project may want to have an Google Cloud account maintained by the community.

Or, we may want to test with a kind of compatible local storage, but note that they may not be compatible.