Use syscall.Exec instead of cmd.Run in non-windows environment

[Songmu]

Using syscall.Exec makes the process tree simpler and is useful when you want the child process to handle the signal.

For example, the direnv program internally uses syscall.Exec.

https://github.com/direnv/direnv/blob/5b8f7d0ade4248e04899015e8628408b6f4becf3/cmd_exec.go#L67

How about it?