Don't include vagrant in shell.nix if running in a container.

It is unlikely that a container will be able to run VirtualBox, so don't bother including vagrant in the nix-shell environment.

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.)

This is an alternative to !164 (closed) that doesn't use a non-default vagrant package.