copykey.go

func (p *Project) insertSshkey() {

	homeDir := fileUtil.FindUserHomeDir()

	cmd := exec.Command("cat", homeDir+"/.ssh/"+p.sshkey.name+".pub", "|", "ssh", p.projectname.name+"@"+p.host.name, "'cat", ">>", "~/.ssh/authorized_keys'")

	err := cmd.Run()
	if err != nil {
	  ...
	}
}

Solução:

keyFile, err := os.Open(fileUtil.FindUserHomeDir() + sep + ".ssh" + sep + p.sshkey.name + ".pub")
if err != nil {
    log.Fatal(err)
}

cmd := exec.Command("ssh", p.projectname.name+"@"+p.host.name, "cat >> ~/.ssh/authorized_keys")
cmd.Stdin = keyFile

out, err := cmd.CombinedOutput()
if err != nil {
    fmt.Println(string(out))
    log.Fatal(err)
}