func replaceEnv(cmd *exec.Cmd, key, value string) {
	if cmd.Env == nil {
		cmd.Env = cmd.Environ()
	}
	cmd.Env = append(cmd.Env, key+"="+value)
}

// appendExperimentEnv appends comma-separated experiments to GOEXPERIMENT.
func appendExperimentEnv(cmd *exec.Cmd, experiments []string) {
	if cmd.Env == nil {
		cmd.Env = cmd.Environ()
	}
	exps := strings.Join(experiments, ",")
	for _, evar := range cmd.Env {