Restore the _legacy_no_dict_keys_sorting branch in Pickler._batch_setitems

[ebarkhordar]

Root cause

Pickler._legacy_no_dict_keys_sorting lost its only reader in #7817. That PR rewrote _batch_setitems to accept the argument Python 3.14 passes, and in the same hunk dropped the early return:

-    def _batch_setitems(self, items):
-        if self._legacy_no_dict_keys_sorting:
-            return super()._batch_setitems(items)
+    def _batch_setitems(self, items, *args, **kwargs):

_check_legacy_cache2 still patches the flag on to rebuild the 2.15.0 config_id, so that lookup now hashes with sorted dict keys and no longer matches the directory 2.15.0 wrote. The flag appears twice in src/, as a definition and as that patch.object write, and nothing reads it.

The fix

Restore the branch, keeping the *args/**kwargs passthrough #7817 added so the 3.14 fix is preserved on both paths.

Verification

• The two new legacy tests fail on main (assert [('a', 2), ('b', 1)] == [('b', 1), ('a', 2)]) and pass with this change; the third pins the default order-insensitive behaviour and passes either way.
• pytest tests/test_fingerprint.py: 24 passed / 9 skipped before, 26 passed / 9 skipped after, in a clean container on Python 3.11.15 with dill 0.4.1.
• With the fix, Hasher.hash({"train": ["train.csv"], "test": ["test.csv"]}) under the patched flag returns 711511d8f1d9bc25, which is the value datasets==2.15.0 actually produces for that object. On main it returns the sorted-key hash cfd3b51f0f8e9fd8. 4.4.0 is the first release where that regressed, matching git tag --contains f7c8e46ec.
• Not checked: I did not build a 2.15.0 cache directory and load it under HEAD. The hash mismatch is measured; the effect on the directory lookup is read from _check_legacy_cache2.

If you would rather retire the 2.14/2.15 compat path than restore it, that is the alternative I raised in the issue and I am happy to send that instead.

Fixes #8410