ssh/config can't be set to a file path

https://kamal-deploy.org/docs/configuration/ssh/#config explains that `ssh/config` can be either a boolean or a path to a file.

Kamal v2.8.1 fails during `setup` command when `ssh/config` is set to a file path with:

```sh
kamal setup
  Finished all in 0.0 seconds
  ERROR (Kamal::ConfigurationError): ssh/config: should be a boolean
```