Fix stamping for rules that don't have a stamp attribute (#3829)

Rules that don't have a `stamp` attribute, for example
`rust_cc_proto_library_aspect`, should not be treated as stamped. This
was broken by #3816 - prior
to that, we were returning false because they also don't have a
`_stamp_flag ` attribute.

By returning true for `rust_cc_proto_library_aspect`, we're
unnecessarily including the volatile stamp files as action inputs, which
harms build caching.

---------

Co-authored-by: Krasimir Georgiev <krasimir@google.com>

Author: justinhorvitz

rust/private/stamp.bzl

@@ -10,7 +10,7 @@ def is_stamping_enabled(ctx, attr):
     Returns:
         bool: The stamp value
     """
-    stamp_num = getattr(attr, "stamp", -1)
+    stamp_num = getattr(attr, "stamp", 0)
     if stamp_num == 1:
         return True
     elif stamp_num == 0: