Ubuntu 解决 public key is not available 问题 NO_PUBKEY
问题： The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5DC22404A6F9F1CA
解决方法：
把pubkey后面的8为复制
gpg --keyserver keyserver.ubuntu.com --recvA6F9F1CA
gpg --export --armor A6F9F1CA | sudo apt-key add -
