Ubuntu20.04 更换国内（阿里云）镜像源

1. 备份原来的源文件

sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak

2. 写入阿里云源信息

sudo tee /etc/apt/sources.list <<-'EOF'
# 阿里镜像源
deb http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse 
deb http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse 
deb http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse 
deb http://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse 
deb http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse 
deb-src http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse 
deb-src http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse 
deb-src http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse 
deb-src http://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse 
deb-src http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
EOF