cli: Match kubectl behavior for KUBECONFIG

A little known feature of kubectl is that it allows to define your KUBECONFIG like this:

```
/some/path/config.yaml:/other/path/other.yaml:/more/paths/here.yaml
```

It reads all specified paths separated by a colons, merges them, and finally uses the resulting merge configuration to talk to the cluster. It would be great if the flux-operator CLI would match this behavior.