Go の `exec.Cmd` を使用して stdout をファイルにリダイレクトするにはどうすればよいですか?

package main

import (
    "os"
    "os/exec"
)

func main() {

    // Open the out file for writing
    outfile, err := os.Create("./out.txt")
    if err != nil {
        panic(err)
    }
    defer outfile.Close()

    // Create the command and assign the outfile to its Stdout
    cmd := exec.Command("echo", "'WHAT THE HECK IS UP'")
    cmd.Stdout = outfile

    // Start the command and wait for it to finish
    err = cmd.Start(); if err != nil {
        panic(err)
    }
    cmd.Wait()
}