release: Add hetznerBootstrap as a build product.

This is in preparation to implement the last task which is left in this
branch (it's actually a nice-to-have rather than a real task):

 - Let the target machine fetch hetzner-nixops-installer from Hydra (if
   available) to speed up deployment.

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