example-robot-data: add preCheck

fix 43 occurences of eg.:
> ValueError: Mesh package://example-robot-data/robots/ur_description/meshes/ur5/collision/base.stl could not be found.

Following 0c0b3af793, pinocchio has
hpp-fcl support, so example-robot-data try to load meshes.

But for `package://example-robot-data/robots/…/mesh.stl` paths to be
resolved by the mesh loader, there must be a
`example-robot-data/robots` folder.

pkgs/development/libraries/example-robot-data/default.nix

@@ -33,6 +33,10 @@ stdenv.mkDerivation (finalAttrs: {
   ];
 
   doCheck = true;
+  # The package expect to find an `example-robot-data/robots` folder somewhere
+  # either in install prefix or in the sources
+  # where it can find the meshes for unit tests
+  preCheck = "ln -s source ../../${finalAttrs.pname}";
   pythonImportsCheck = [
     "example_robot_data"
   ];