替换/etc/apt/source.list文件内容为：

deb http://mirrors.aliyun.com/ubuntu-ports/ focal main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu-ports/ focal main restricted universe multiverse

deb http://mirrors.aliyun.com/ubuntu-ports/ focal-security main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu-ports/ focal-security main restricted universe multiverse

deb http://mirrors.aliyun.com/ubuntu-ports/ focal-updates main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu-ports/ focal-updates main restricted universe multiverse

deb http://mirrors.aliyun.com/ubuntu-ports/ focal-proposed main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu-ports/ focal-proposed main restricted universe multiverse

deb http://mirrors.aliyun.com/ubuntu-ports/ focal-backports main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu-ports/ focal-backports main restricted universe multiverse