Disable Gnome Keyring SSH_AUTH_SOCK

To effectively use a Yubikey as GPG/SSH key in Ubuntu, you will need to disable the ssh daemon built in to gnome-keyring.

To do this, add a line Hidden=true to the relevant config:

(cat /etc/xdg/autostart/gnome-keyring-ssh.desktop; echo Hidden=true) > ~/.config/autostart/gnome-keyring-ssh.desktop

(Source: https://askubuntu.com/a/607563)