envy.go
func MustSet(key string, value string) error {
	gil.Lock()
	defer gil.Unlock()
	err := os.Setenv(key, value)
	if err != nil {
		return err
	}
	env[key] = value
	return nil
}
func MustSet(key string, value string) error {
	gil.Lock()
	defer gil.Unlock()
	err := os.Setenv(key, value)
	if err != nil {
		return err
	}
	env[key] = value
	return nil
}
// MustSet the value into the underlying ENV, as well as envy.
// This may return an error if there is a problem setting the
// underlying ENV value.