[blender] Provide script's relative path with ${NODE_SOURCECODE_FOLDER}

This prevents having an absolute path for the script, which led to errors
when opening, for example, a project in another environment.

Author: cbentejac

meshroom/nodes/blender/ScenePreview.py

@@ -32,7 +32,7 @@ class ScenePreview(desc.CommandLineNode):
             name="script",
             label="Script",
             description="Path to the internal script for rendering in Blender.",
-            value=os.path.join(currentDir, "scripts", "preview.py"),
+            value="${NODE_SOURCECODE_FOLDER}/scripts/preview.py",
             invalidate=False,
             group="",
             advanced=True,