`${default}` not interpolated into `help` if using vars

```go
type cli struct {
  Config string `type:"path" default:"${config_file}" help:"Default: ${default}"`
}

func main() {
  kong.Parse(&cli,
    kong.Vars{
      "config_file": "~/.app.conf",
    })
}
```

This prints in help `Default: ${config_file}` and not `Default: ~/.app.conf`.