Add an `$` at the end of the `grep` regex. Without it, `checkRef foo`
would always imply `checkRef foo.drv`. We want to tell these situations
apart to more precisely test what is going on.
This is requires if you have attribute names with dots in them. So
you can now say:
$ nix-instantiate '<nixos>' -A 'config.systemd.units."postgresql.service".text' --eval-only
Fixes#151.