fix(functions): use golang image for CI/CD example
# [START functions_ci_cd_cloud_build]
steps:
-- name: 'mirror.gcr.io/library/golang'
+- name: golang
args: ['go', 'version']
-- name: 'mirror.gcr.io/library/golang'
+- name: golang
args: ['go', 'test']
dir: 'function/dir/from/repo/root'
- name: 'gcr.io/cloud-builders/gcloud'
