PPA地址: launchpad.proxy.ustclug.org

    1. 修改 /etc/apt/sources.list.d 下面需要代理的仓库地址，将 ppa.launchpad.net 换成 launchpad.proxy.ustclug.org
    2. 注意, http 改为 https

    例如, ubuntu20, 截图软件 shutter 的源, 改为

    1. deb https://launchpad.proxy.ustclug.org/linuxuprising/shutter/ubuntu focal main
    2. # deb-src https://launchpad.proxy.ustclug.org/linuxuprising/shutter/ubuntu focal main