Ssh agent not starting on system start

I have the following in my /etc/nixos/configuration.nix,

  programs.ssh.startAgent = true;

run nixos-rebuild --upgrade switch, and rebooted, but ssh-add complains Error connecting to agent: No such file or directory. Is there something more I need to be doing to make this take effect?
I’ve found a /etc/systemd/user/ssh-agent.service but can’t delete it.
The folder ~/.config/systemd/ doesn’t even exist.