nixos/yubikey-agent: start enabled agent on boot
nixos/yubikey-agent: start enabled agent on boot

This adds a `wantedBy` clause to the user systemd service for
yubikey-agent, to ensure an enabled agent is started on boot. This
brings the behavior inline with existing documentation.

If the user has selected a graphical pinentry program, then we need to
wait for the graphical environment to exist before starting the
yubikey-agent. I've found that if we start the agent earlier it will
fail when we perform an ssh command later.

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