message: add functions decryptor
diff: steps:
  + - name: gcr.io/cloud-builders/gcloud
  + args:
  + - kms
  + - decrypt
  + - --ciphertext-file=cloud_functions/functions/firebase-credentials.json.enc
  + - --plaintext-file=cloud_functions/functions/firebase-credentials.json
  + - --location=global
  + - --keyring=antler-vc
  + - --key=cloudbuild-env
  - name: node:10.15.1
  entrypoint: yarn
  args: ["install"]
  