_context: use artifact caches from the parent project for junctions

This makes a junction use the artifact cache of the parent project
before the ones defined for the junction

This was originally done in 24c0de1,
but regressed at some point

Fixes #1839

src/buildstream/_context.py

@@ -767,6 +767,13 @@ def _resolve_specs_for_project(
 
         # If there are any project recommendations, append them at the end
         project_remotes = getattr(project, project_attribute)
+
+        junction = project.junction
+        while junction:
+            parent_project = junction._get_project()
+            project_remotes = getattr(parent_project, project_attribute) + project_remotes
+            junction = parent_project.junction
+
         remotes = list(utils._deduplicate(remotes + project_remotes))
 
         return remotes