Expand the scope of tidb_allow_fallback_to_tikv

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

1. create table a
2. alter table a set replica 1
3. lightning local-backend 1TB csv files
4. wait lightning print 'import completed' log
5. read tiflash engine:select count(*) from a;

### 2. What did you expect to see? (Required)
1 row, 0 warings
### 3. What did you see instead (Required)
| ERROR 1105 (HYQ00): [FLASH:Coprocessor:RegionError] Region 131439251 is unavailable: (while doing learner read for table, table_id: 1983)

### 4. What is your TiDB version? (Required)
v5.4.0