Running datasette publish cloudrun now shows this message:

Please choose a target platform:
[1] Cloud Run (fully managed)
[2] Cloud Run on GKE
[3] a Kubernetes cluster
[4] cancel
Please enter your numeric choice: 1

To specify the platform yourself, pass --platform managed. Or, to make this the default target platform, run gcloud config set run/platform managed.

May as well set that as a default.