首先报错内容：
Errors during downloading metadata for repository 'AppStream':
- Status code: 404 for https://repo.huaweicloud.com/centos/8/AppStream/x86_64/os/repodata/repomd.xml (IP: 111.206.179.18)
Error: Failed to download metadata for repo 'AppStream': Cannot download repomd.xml: Cannot download repodata/repomd.xml: All mirrors were tried
由于现在centos8停止维护了，其实就是镜像里边的$releasever替换成8-stream。
cd /etc/yum.repos.d
tar -zcf bak20220408.tar.gz ./*.repo
rm -rf ./*.repo
wget -O /etc/yum.repos.d/CentOS-Base.repo https://repo.huaweicloud.com/repository/conf/CentOS-8-reg.repo
# 把 CentOS-Base.repo 里 的baseurl所有 $releasever 替换成 8-stream，然后执行
yum clean all && yum makecache
