2configs.yubikey: fix gpg-agent.conf rendering for enable-ssh-support
Newer nixpkgs renders 'settings.enable-ssh-support = true' as
'enable-ssh-support true', which gpg-agent rejects with 'argument not
expected' since it's a boolean flag. Use "" instead so the keyValue
formatter emits a bare 'enable-ssh-support ' line gpg-agent accepts.