I have two environment variables. One is `TF_VAR_UN` and another is `TF_VAR_PW`. Then I have a terraform file that looks like this.

```
resource "google_container_cluster" "primary" {
    name = "marcellus-wallace"
    zone = "us-central1-a"
    initial_node_count = 3

    master_auth {
        username = ${env.TF_VAR_UN}
        password = ${env.TF_VAR_PW}
    }

    node_config {
        oauth_scopes = [
            "https://www.googleapis.com/auth/compute",
            "https://www.googleapis.com/auth/devstorage.read_only",
            "https://www.googleapis.com/auth/logging.write",
            "https://www.googleapis.com/auth/monitoring"
        ]
    }
}

```

The two values I'd like to replace with the environment variables `TF_VAR_UN` and `TF_VAR_PW` are the values username and password. I tried what is shown above, with no success, and I've toyed around with a few other things but always get syntax issues.