Changes

fix(pipelines): make tiflash release-8.5 ut cache prepare idempotent (#4384)

## Summary
- backport the `build_cache_ready` fallback check used in `latest` to
`release-8.5` pull_unit_test pipeline
- make `tests/.build` creation idempotent by changing `mkdir
tests/.build` to `mkdir -p tests/.build`

## Root Cause
In `release-8.5` pull_unit_test, `build_cache_ready` may remain `false`
when cache is restored, so the script enters the cache-miss branch and
runs `mkdir tests/.build`. If the directory already exists from restored
cache, the stage fails with `File exists`.

## Replay Test
-
https://do.pingcap.net/jenkins/blue/organizations/jenkins/pingcap%2Ftiflash%2Frelease-8.5%2Fpull_unit_test/detail/pull_unit_test/356/pipeline/
 (commit: 0eba514)