backintime-qt4: package is broken

the install phase writes into /nix :

```
<snip>
install -d
/nix/store/1kc8xcni0wp4y35vafh03rdxvqkrsxvl-backintime-qt4-1.1.24/../etc/dbus-1/system.d
install --mode=644 net.launchpad.backintime.serviceHelper.conf
/nix/store/1kc8xcni0wp4y35vafh03rdxvqkrsxvl-backintime-qt4-1.1.24/../etc/dbus-1/system.d
<snap>
```