Changes

Fix tici pipeline (#4111)

When rerun test with cache, tiflash link has a bug, after
https://github.com/PingCAP-QE/ci/pull/4108/files merged

```
+ mv tiflash tiflash_dir
mv: 'tiflash' and 'tiflash_dir/tiflash' are the same file
script returned exit code 1
```
 (commit: a9e2715)