nixos/stage-1: set up /dev/fd

Otherwise, stage-2-init.sh will complain about not having access to
/dev/fd/62 as of systemd v246.

On IRC, flokli said:

    15:14 <flokli> cole-h: hmmm... I could imagine some of the setup inside /dev has been moved into other parts of systemd
    15:14 <flokli> And given we run systemd much later (outside initramfs only) it doesn't work properly here
    15:17 <flokli> We probably don't invoke udev correctly

nixos/modules/system/boot/stage-1-init.sh

@@ -218,6 +218,7 @@ done
 # Create device nodes in /dev.
 @preDeviceCommands@
 echo "running udev..."
+ln -sfn /proc/self/fd /dev/fd
 mkdir -p /etc/systemd
 ln -sfn @linkUnits@ /etc/systemd/network
 mkdir -p /etc/udev