networking: Fix timeout when `lo` has aliased IPs

With a config like

    {
      networking.interfaces."lo".ip4 = [
        { address = "10.8.8.8"; prefixLength = 32; }
      ];
    }

a nixos-rebuild switch would take a long time, and you'd see:

    $ systemctl list-jobs
       JOB UNIT                                TYPE  STATE
    734400 network-interfaces.target           start waiting
    734450 sys-subsystem-net-devices-lo.device start running
    734449 network-link-lo.service             start waiting

and:

    systemd[1]: sys-subsystem-net-devices-lo.device: Job sys-subsystem-net-devices-lo.device/star>
    systemd[1]: sys-subsystem-net-devices-lo.device: Job sys-subsystem-net-devices-lo.device/star>
    systemd[1]: Timed out waiting for device sys-subsystem-net-devices-lo.device.

This removes the device dependency for `lo` and fixes this bug.

Closes #7227