nixos/tests/installer: Add lndir to extraDeps

In commit 17dd7bcd

 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 <aszlig@nix.build>
Cc: @jtojnar, @xeji, @edolstra