hydra-slave: Add SSH pubkey for buildfarm@nixos.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
hydra-slave: Add signing-key.pub from NixOS.

According to @edolstra (not literally):

Otherwise nix-copy-closure to that machine will fail to check the
signature.

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