signal-desktop: Fix a possible crash when saving an attachment

If executed in a pure environment (nix-shell --pure) or depending on the
`gtk3` version of the system Signal-Desktop was e.g. crashing when
clicking on a PDF attachment (instead of showing the dialog to save a
file).
Using wrapGAppsHook and setting XDG_DATA_DIRS to the correct version
fixes this bug.

The error message was the following:
```
(signal-desktop:30756): Gtk-WARNING **: 14:04:49.073: Could not find the icon 'user-home-symbolic-ltr'. The 'hicolor' theme
was not found either, perhaps you need to install it.
You can get a copy from:
        http://icon-theme.freedesktop.org/releases

(signal-desktop:30756): GLib-GIO-ERROR **: 14:04:49.134: No GSettings schemas are installed on the system
Trace/breakpoint trap
```