DO NOT MERGE: Bumping artifact version

This commit is intended to trigger a full rebuild for a test of this branch.
diff --git a/buildstream/_versions.py b/buildstream/_versions.py
index 4b1bc7e..28f00f8 100644
--- a/buildstream/_versions.py
+++ b/buildstream/_versions.py
@@ -33,4 +33,4 @@
 # or if buildstream was changed in a way which can cause
 # the same cache key to produce something that is no longer
 # the same.
-BST_CORE_ARTIFACT_VERSION = 2
+BST_CORE_ARTIFACT_VERSION = 3