exec: Stdout already set

```
type CmdRunner struct {
logger hclog.Logger
cmd *exec.Cmd

stdout io.ReadCloser
stderr io.ReadCloser

// Cmd info is persisted early, since the process information will be removed
// after Kill is called.
path string
pid int

addrTranslator
}
```
When executing the kill command to terminate a child process, I found that the runner did not close the stdout and stderr.