centos8更换国内源
1备份旧的配置文件
mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
2下载新的 CentOS-Base.repo 到 /etc/yum.repos.d/
wget -O /etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-8.repo
3替换部分字段（非阿里云机器需要做）
sed -i -e ‘/mirrors.cloud.aliyuncs.com/d’ -e ‘/mirrors.aliyuncs.com/d’ /etc/yum.repos.d/CentOS-Base.repo
4生成缓存
yum makecache