BR cannot restore when max-index-length>3072

[3pointer]

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

   1. set tidb config max-index-length to 12288;
   2. create table t1 (a varchar(3072) primary key);
   3. br backup full -s local:///home/tidb/tmp/br
   4. drop table t1;
   5. br restore full -s local:///home/tidb/tmp/br

2. What did you expect to see?
   restore success

3. What did you see instead?
   Error: [ddl:1071]Specified key was too long; max key length is 3072 bytes

4. What version of BR and TiDB/TiKV/PD are you using?
   br master
   tidb master