まんまこれ
cd /tmp/
wget http://mirrors.kernel.org/ubuntu/pool/main/o/openssh/openssh-client_7.2p2-4ubuntu2.6_amd64.deb
dpkg -x openssh-client_7.2p2-4ubuntu2.6_amd64.deb /tmp/deb
sudo mv /usr/bin/ssh-agent /usr/bin/ssh-agent.18.04
# for safekeeping in case of bionic updates
sudo mv /tmp/deb/usr/bin/ssh-agent /usr/bin/ssh-agent.16.04
sudo cp /usr/bin/ssh-agent.16.04 /usr/bin/ssh-agent
sudo chown root:ssh /usr/bin/ssh-agent