Unstable test in default_as_expression.test

[zimulala]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

./run-tests.sh -b Y -r ddl/default_as_expression

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

alter table t0 add column c2 date default (date_format(now(),'%Y-%m'));
Error 1292 (22007): Incorrect datetime value: '2024-05'

3. What did you see instead (Required)

alter table t0 add column c2 date default (date_format(now(),'%Y-%m'));
Error 1292 (22007): Incorrect datetime value: '2024-04'

4. What is your TiDB version? (Required)

v8.1