Add XPalm back into dependencies' downstream tests

[Samuel-amap]

Tests are currently a little unstable at times, causing ambiguity in dependencies that test XPalm as a downstream test.
While there are other options, given that XPalm isn't released yet and is a 'leaf package', it's somewhat convenient to remove it from downstream tests until things stabilise further.

This is just a reminder to add the repository/package back to the list of packages tested in downstream tests. Check Integration.yml in the .github folder for PlantGeom/MultiScaleTreeGraph/PlantBioPhysics/PlantMeteo/PlantSimEngine.

(There are a couple of benchmarks in PlantSimEngine in a downstream.yml folder, those are distinct tests executed independently that don't have those stability issues and don't need to be affected).