desktop-file-service: doesn't properly watch applications directories on NixOS

### Expected Behavior

Anytime I add or remove an application from my system the application menu should reflect that.

### Current Behavior
On [NixOS](https://nixos.org) any application that is installed into the system environment that has a
`share/` directory gets added to the `XDG_DATA_DIRS` environment variable.

So take note that it will be populated with a bunch of absolute paths, possibly symlinks, and anytime the system is rebuilt paths will be changed and **added**.

I don't think the current `desktop-file-service` can properly watch those changes, as anytime a new application is installed (new path added to XDG_DATA_DIRS) it doesn't update.

This requires our users to log out and log in to see the appropriate effect.