Changes

feat(tiflash): cache build for release-7.5 PR pipelines (#4130)

## What

Backport the TiFlash PR build-cache optimization to the release-7.5 pull
pipelines to avoid rebuilding the same commit SHA on retried runs.

## Changes

- Add build cache probe + stage gating to release-7.5 TiFlash PR
integration pipeline
- Add build cache probe + stage gating + cached binary restore for unit
pipeline
- Use stable cache keys via `prow.getCacheKey('tiflash', REFS,
'it-build'/'ut-build')`

## Notes

This mirrors the optimization introduced in commit 9e3c33e for the
equivalent latest pipelines.

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Co-authored-by: wuhuizuo <wuhuizuo@126.com>
 (commit: 56986e9)