ubuntu20.04换清华源后不能更新、https更新失败

务必注意时效性！
日期：2021.12.26

原因与官方解决方案：

https://github.com/tuna/issues/issues/1342

快速解决问题

1. 换回默认的官方源
2. 更新/安装 ca-certificates

ml@ml-virtual-machine:/etc/apt$ sudo apt install ca-certificates
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages will be upgraded:
  ca-certificates
...
done.

3. 然后换上清华源，不出意外的话就能用了

ml@ml-virtual-machine:/etc/apt$