错误：
Error: Failed to download metadata for repo 'appstream': Cannot prepare internal mirrorlist: No URLs in mirrorlist
解决办法：
To migrate from CentOS 8 to CentOS Stream 8, run the following commands:
dnf --disablerepo '*' --enablerepo=extras swap centos-linux-repos centos-stream-repos
然后执行
dnf distro-sync