How to save cookies as a file in Golang

package main

import (
	"fmt"
	"log"
	"net/http"
	"net/url"
	"os"
	"strings"

	"github.com/PuerkitoBio/goquery"
	"github.com/juju/persistent-cookiejar"
	"golang.org/x/term"
)

func main() {
	jar, _ := cookiejar.New(nil)
	http.DefaultClient.Jar = jar
	defer jar.Save()

	// If the file exists, check whether logged in
	_, err := os.Stat(cookiejar.DefaultCookieFile())
	if err == nil {
		doc, _ := getDocument("https://atcoder.jp/home")
		navbarRight := doc.Find("div#navbar-collapse > ul.navbar-right")
		if navbarRight.Children().Length() == 2 {
			fmt.Println("Already logged in!")
			return
		}
	}

	var username string
	fmt.Print("Username: ")
	fmt.Scan(&username)

	fmt.Print("Password: ")
	// Don't press ctrl+c while Go reads password because your terminal won't display any inputs after that.
	// To resolve it, see gist.github.com/montanaflynn/5ae3eeae7212b0ba232f46e88f1ab67f
	bypePassword, _ := term.ReadPassword(int(os.Stdin.Fd()))

	loginUrl := "https://atcoder.jp/login"
	doc, _ := getDocument(loginUrl)
	token, found := doc.Find(`form input[type="hidden"]`).Attr("value")
	if !found {
		log.Fatal("error: cannot find CSRF token")
	}

	values := url.Values{
		"username":   {username},
		"password":   {string(bypePassword)},
		"csrf_token": {token},
	}
	req, _ := http.NewRequest("POST", loginUrl, strings.NewReader(values.Encode()))
	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")

	resp, err := http.DefaultClient.Do(req)
	if err != nil {
		log.Fatal(err)
	}
	defer resp.Body.Close()

	if resp.Request.URL.String() == loginUrl {
		log.Fatal("Failed to login. Check your username/password")
	}

	fmt.Println("Successfully logged in!")
}

func getDocument(url string) (*goquery.Document, error) {
	resp, err := http.Get(url)
	if err != nil {
		log.Fatal(err)
	}
	defer resp.Body.Close()

	return goquery.NewDocumentFromReader(resp.Body)
}