update gcloud commands to --decode base64

[mvansoens]

For the article https://circleci.com/docs/2.0/google-auth/, the following block should be updated:

From:

echo $GCLOUD_SERVICE_KEY | sudo gcloud auth activate-service-account --key-file=-
sudo gcloud --quiet config set project ${GOOGLE_PROJECT_ID}
sudo gcloud --quiet config set compute/zone ${GOOGLE_COMPUTE_ZONE}
to:

echo $GCLOUD_SERVICE_KEY | base64 --decode | sudo gcloud auth activate-service-account --key-file=-
sudo gcloud --quiet config set project ${GOOGLE_PROJECT_ID}
sudo gcloud --quiet config set compute/zone ${GOOGLE_COMPUTE_ZONE}

without --decoding the base64 encoded json file, an error is thrown because of --key-file= inconsistent format.