GH CD: try to see content of workdir.

Beremiz revision bump

GH CD: Merge sources and snap dir instead of just giving checkout and artfifact download the same path

Update README.md

GH CD: attempt to preserve content of build/source by swapping checkout and artifact download steps.

Fix generation of revisions.txt in makefile

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.

Beremiz revision bump

GH CD: Add Modbus and open62541 sources.

Add new sanity checksums.

SNAP: add Modbus source and dependencies

SNAP: add SVGHMI and OPC-UA dependencies

Add (untested) snapcraft workflow for GitHub action. Add documentation about manual snapcraft invokation.

Add Snap Package.

For now :
- using makefile :
$ cd ~/build; make -f ~/src/beremiz_public_dist/Makefile DIST=snap
- build and run of python exemple was tested
- no fieldbus support yet.

GH CI : add job 'needs' to ensure artifact is ready

GH CI : use artifacts to get data flowing from one workflow to the other