Adding unique index with global sort gets stuck

[tangenta]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

set global tidb_cloud_storage_uri = 'xx';
set global tidb_enable_dist_task = on;
alter table xx add unique index idx(xxx);

2. What did you expect to see? (Required)

No error.

3. What did you see instead (Required)

DDL stuck.

4. What is your TiDB version? (Required)

af89779