artifactcache: ensure method is called from main process

Ensure that add_artifact_size() is called from the main process.

buildstream/_artifactcache/artifactcache.py

@@ -272,6 +272,8 @@ class ArtifactCache():
     #     artifact_size (int): The size to add.
     #
     def add_artifact_size(self, artifact_size):
+        assert utils._is_main_process()
+
         self._cache_size = self.get_cache_size() + artifact_size
         self._write_cache_size(self._cache_size)