SNAP: replace lxml versions to be compatible with Beremiz.

Forcing ubuntu20 python-lxml (4.5.0) package instead of PIP vesrion (4.9.1).
4.9.1 leads to derived variable types to appear as "derived" instead of real type name.
Problem happens with plcopen/variables_infos.ysl2 and plcopen/VariableInfoCollector.py.
No clue for now about what really breaks.

[Desktop Entry]
Type=Application
Name=Beremiz IDE
Comment=IEC-61131 Open Source PLC programming
Exec=beremiz.ide %f
Icon=${SNAP}/meta/gui/beremiz.png