_yaml.py: Only retrieve provenance in node_get() when needed

We were indiscriminately retrieving the node's provenance data in the
`node_get()` function which was accounting for approximately a third of
the total runtime of `node_get()` which dominates pre-scheduler time in
`bst build`.  This change ameliorates that situation by only retrieving
the provenance data when it's actually needed.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

buildstream/_yaml.py

@@ -364,8 +364,8 @@ _sentinel = object()
 #
 def node_get(node, expected_type, key, indices=None, default_value=_sentinel):
     value = node.get(key, default_value)
-    provenance = node_get_provenance(node)
     if value is _sentinel:
+        provenance = node_get_provenance(node)
         raise LoadError(LoadErrorReason.INVALID_DATA,
                         "{}: Dictionary did not contain expected key '{}'".format(provenance, key))