Revert "_project.py: Bumping artifact version for execution environment"

This reverts commit cd62e98834d91cbf32ac7d1408b6c0972c690591.
diff --git a/buildstream/element.py b/buildstream/element.py
index d03f32c..86cac56 100644
--- a/buildstream/element.py
+++ b/buildstream/element.py
@@ -52,7 +52,7 @@
 # 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 = 1
+_BST_CORE_ARTIFACT_VERSION = 0
 
 
 # _KeyStrength():