需求场景
树莓派装了ubuntu mate桌面系统，替换为国内ubuntu源报错。
报错原因
清华，阿里等镜像源中ubuntu不包含arm源，arm源在ubuntu-ports中。
解决方案
ubuntu替换为ubuntu-ports即可
# 清华源ubuntu20.04
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ focal main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ focal-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ focal-backports main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ focal-security main restricted universe multiverse