add index for system tables takes a long time while upgrade cluster

[aytrack]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. deploy a v7.5.0 cluster and create a table with 5000 partition
2. truncate the table
3. during truncate the table , upgrade cluster to nightly

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

1. upgrade cluster success

3. What did you see instead (Required)

1. the system table add index takes a long time led tiup upgrade faied.
2. you can see the user ddl are paused, but the system table add index takes about 2min.
   

4. What is your TiDB version? (Required)

v7.5.0