luakit: New expression

There is a small issue regarding `libunique` left: Sometimes (not always), you get the message:

```
(.luakit-wrapped:17645): Unique-DBus-WARNING **: Unable to open a connection to the session bus: /nix/store/riscsd14aj678l1q5dv9jhhd88lc6sg1-dbus-libs-1.6.16/bin/dbus-launch terminated abnormally without any error message

(.luakit-wrapped:17645): Unique-DBus-WARNING **: Unable to connect to the running instance, aborting.
```

Then, luakit starts a new instance, even though another one is already running. Not sure how to fix this.