Install a vagrant package without libvirt support in nix-shell.

Given that the instructions for local development suggest using VirtualBox instead of libvirt, we can install vagrant without pulling in all the libvirt dependencies.

For me, this reduces the amount I need to download to run nix-shell from >1G to <15M. (I've carrying a patch that removed the dependency to avoid downloading that much; the measurements only include the effect of adding vagrant or the minimal vagrant to the definition.)