hetzner-bootstrap: Fix eval for latest <nixpkgs>.

The commit NixOS/nixpkgs@b3cfb90 added a lib argument to the function
attrset of <nixpkgs/nixos/modules/installer/tools/tools.nix>, so we need
to pass it on our side as well.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Merge pull request #210 from aszlig/fix-hetzner-bootstrap

hetzner-bootstrap: Fix evaluation for latest <nixpkgs>.