Define a user-created service account

Instead of use the CloudBuild Service Agent, we use a dedicated service
account for the release process

Signed-off-by: Arnaud Meukam <ameukam@gmail.com>

gcb/stage/cloudbuild.yaml

@@ -77,6 +77,7 @@ steps:
   secretEnv:
   - GITHUB_TOKEN
   - DOCKERHUB_TOKEN
+  serviceAccount: 'projects/k8s-release/serviceAccounts/k8s-relase-sa'
   args:
   - "bin/krel"
   - "stage"