nixos/tests/installer: Add lndir to extraDeps

In commit 17dd7bcd89d568596f52356624be82201ea84779 the systemd unit
generator now uses lndir instead of "ln -s", so we need to have lndir
available in the Nix store for the installer tests to be able to build
the units and thus the whole system.

This should fix the installer tests, although I have only tested it with
the "simple" subtest.

Signed-off-by: aszlig <[e-mail]>
Cc: @jtojnar, @xeji, @edolstra

nixos/tests/installer.nix

···
                 nixos-artwork.wallpapers.gnome-dark
                 perlPackages.XMLLibXML
                 perlPackages.ListCompare
+                xorg.lndir
 
                 # add curl so that rather than seeing the test attempt to download
                 # curl's tarball, we see what it's trying to download