Changes

fix(tiflash): handle ut-build cache restore in unit test prepare (#4345)

## Summary
- make `Unit Test Prepare` resilient when `build_cache_ready` was not
initialized before cache restore
- detect restored ut-build content by checking `tests/.build/tiflash`
inside the cache block
- use `mkdir -p tests/.build` to avoid `File exists` failure

## Why
In pull_unit_test #5456, `Unit Test Prepare` failed with `mkdir: cannot
create directory 'tests/.build': File exists` after cache restore. This
change makes the step idempotent and tolerant to cache state mismatches.

## Replay test

https://do.pingcap.net/jenkins/blue/organizations/jenkins/pingcap%2Ftiflash%2Fpull_unit_test/detail/pull_unit_test/5461/pipeline/
 (commit: ac4e61e)