skaffold build: --kube-context flag does not default correctly.

### Expected behavior
`skaffold build --kube-context=''` and `skaffold build` output the same thing, given that the [cli docs](https://skaffold.dev/docs/references/cli/#skaffold-build) specify that the default for this command is `--kube-context=''`.

### Actual behavior
`skaffold build` seems to take the active context as the kube-context and not `''` as documented.

When I am trying to deploy images to GCR for a later deployment (to Cloud Run), I am not paying attention to what my active context is, but sometimes it is a stopped minikube cluster. I would expect when running `skaffold build` with the implicit default of `--kube-context=''`, it acts as a 'contextless' build that doesn't assume a local build, but I instead get the error...
```
Failed to build the app. Error: creating runner: creating builder: getting docker client: getting minikube env: running [minikube docker-env --shell none]
- stdout: "* The control plane node must be running for this command\n - To fix this, run: \"minikube start\"\n"
- stderr: ""
- cause: exit status 69
```

### Information

- Skaffold version: v1.12.0
- Operating system: macOS v10.15.4