尝试执行do-release-upgrade从 Ubuntu 18.04 升级到 20.04 时，出现以下错误
Checking for a new Ubuntu release
Get:1 Upgrade tool signature [1,554 B]
Get:2 Upgrade tool [1,342 kB]
Fetched 1,343 kB in 0s (0 B/s)
authenticate 'focal.tar.gz' against 'focal.tar.gz.gpg'
Authentication failed
Authenticating the upgrade failed.
There may be a problem with the network or with the server.
仅供参考：我已经试过了sudo apt-get install --reinstall ubuntu-keyring。