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.

# Snap distribution

main_target: Snap

tar_opts=--absolute-names --exclude=.hg --exclude=.git --exclude=.*.pyc --exclude=.*.swp

Snap: snap_built
snap_built: all_sources revisions.txt
	tar -C $(src) $(tar_opts) -c snap revisions.txt | tar -C sources -x
	cd sources;  snapcraft --debug 
	touch $@