_yaml.py: Fixed node_copy()

Jonathan Maw caught this bug, we were forgetting to transport
the regular values to the new copied dictionary here.
diff --git a/buildstream/_yaml.py b/buildstream/_yaml.py
index c8151d5..06a8060 100644
--- a/buildstream/_yaml.py
+++ b/buildstream/_yaml.py
@@ -613,6 +613,8 @@
             copy[key] = list_copy(value)
         elif isinstance(value, Provenance):
             copy[key] = value.clone()
+        else:
+            copy[key] = value
 
     ensure_provenance(copy)