Multi-platform image created with skaffold build is not sent to default-registry

### Expected behavior
When running `skaffold build --platform=linux/arm64,linux/amd64 --default-repo=gcr.io/my-registry`, Skaffold should push the created multi-platform image to the specified default-repo.

### Actual behavior
Instead, it tries to push it to the local Docker engine, which does not support multi-platform images. To make it work, the command should be run with the `--push` flag: `build --platform=linux/arm64,linux/amd64 --default-repo=gcr.io/my-registry --push`

### Information

- Skaffold version: main V2
- Operating system: MacOS, x86
- Installed via: compile from main
- Contents of skaffold.yaml: Use Java guest book example from [here](https://github.com/GoogleCloudPlatform/cloud-code-samples/tree/v1/java/java-guestbook)