nixos/taskserver: Fix reference to certtool.

With the merge of the closure-size branch, most packages now have
multiple outputs. One of these packages is gnutls, so previously
everything that we needed was to reference "${gnutls}/bin/..." and now
we need to use "${gnutls.bin}/bin/...".

So it's not a very big issue to fix.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>