Where we don't have a proper snapd support.
Approach similar to the one used in UDI.
For the case where we have Subiquity snap, but not UDI.
The script added is a stripped version of the UDI one.
Removed code related to:
- GTK and GDK;
- GI repository;
- Xkb;
Preserved:
- Python;
- XDG directory definitions;
- font config;