Correct comparison against stamping policy name

The three valid stamping policies are named as follows:
- Allow
- Force
- Prevent

When checking whether to stamp injected values for tagged fields, we
were looking mistakenly for a value of "Never", which was the previous
name for the policy now known as "Prevent". Correct that sentinel
value to allow rule callers to prevent stamping of injected values.

cue/cue.bzl

@@ -33,7 +33,7 @@ CUEInstanceInfo = provider(
 
 def _replacer_if_stamping(stamping_policy):
     # NB: We can't access the "_cue_config" attribute here.
-    return Label("//tools/cmd/replace-stamps") if stamping_policy != "Never" else None
+    return Label("//tools/cmd/replace-stamps") if stamping_policy != "Prevent" else None
 
 def _add_common_source_consuming_attrs_to(attrs):
     attrs.update({