粗错信息：
    CentOS-8 - AppStream 6.7 B/s | 38 B 00:05
    Error: Failed to download metadata for repo ‘AppStream’: Cannot prepare internal mirrorlist: No URLs in mirrorlist
    阿里云官方解释是centos8官方源已下线，建议切换centos-vault源
    1、删除AppStream源
    rm -f /etc/yum.repos.d/CentOS-AppStream.repo
    2、备份旧的Base源
    mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
    3、下载Vault源
    curl -o /etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-vault-8.5.2111.repo
    4、清除缓存和生成缓存
    yum clean all && yum makecache