_project.py: 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

Fixes apache#401

buildstream/_artifactcache.py

@@ -1046,8 +1046,5 @@ def _get_cache_volume_size(self):
 #   A list of ArtifactCacheSpec instances describing the remote artifact caches.
 #
 def _configured_remote_artifact_cache_specs(context, project):
-    project_overrides = context.get_overrides(project.name)
-    project_extra_specs = ArtifactCache.specs_from_config_node(project_overrides)
-
     return list(utils._deduplicate(
-        project_extra_specs + project.artifact_cache_specs + context.artifact_cache_specs))
+        project.artifact_cache_specs + context.artifact_cache_specs))

buildstream/_project.py

@@ -549,7 +549,15 @@ def _load_second_pass(self):
         #
 
         # Load artifacts pull/push configuration for this project
-        self.artifact_cache_specs = ArtifactCache.specs_from_config_node(config, self.directory)
+        project_specs = ArtifactCache.specs_from_config_node(config, self.directory)
+        override_specs = ArtifactCache.specs_from_config_node(
+            self._context.get_overrides(self.name), self.directory)
+
+        self.artifact_cache_specs = override_specs + project_specs
+
+        if self.junction:
+            parent = self.junction._get_project()
+            self.artifact_cache_specs = parent.artifact_cache_specs + self.artifact_cache_specs
 
         # Load remote-execution configuration for this project
         project_specs = SandboxRemote.specs_from_config_node(config, self.directory)