pipeline: make them "exportable"

Add a new `export` property to the `Pipeline` object that indicates
whether a the result, i.e. the tree after the pipelines has been
built, should be exported, i.e. copied to the output directory.
In the current format (v1), the main pipeline, gets marked as such
by the corresponding loader.

osbuild/formats/v1.py

@@ -76,6 +76,7 @@ def load_pipeline(description: Dict, index: Index, manifest: Manifest) -> Pipeli
         asm.inputs = {
             "tree": Input(info, {"pipeline": {"id": pipeline.tree_id}})
         }
+        pipeline.export = True
 
     return pipeline