Dataflow run fails when service account is not set.

The dataflow runner is not setting the service account for the job that is being set up. This causes failures when trying to deploy.

```
 Workflow failed. Causes: There was a problem refreshing your credentials. Please check:
1. Dataflow API is enabled for your project.
2. There is a robot service account for your project:
service-[project number]@dataflow-service-producer-prod.iam.gserviceaccount.com should have access to your project. If this account does not appear in the permissions tab for your project, contact Dataflow support.
```

Adding a flag to set the service account will fix this issue.