Changes

opt(pipelines/pingcap/tidb): speedup set build desc (#3681)

This pull request refactors how the PR description is set in all CI
pipeline scripts for the `pingcap/tidb` repository's release-6.1 and
release-6.2 branches. The main change is moving the call to
`prow.setPRDescription(REFS)` from inside a container script block to
the top-level script context, right after reading the PR references.
This ensures the PR description is set earlier and more reliably across
all pipeline jobs.

Refactoring PR description setup:

* Moved the `prow.setPRDescription(REFS)` call to immediately after
reading PR references (`REFS`) at the top of each pipeline script,
instead of inside the `net-tool` container block. This affects all major
job scripts for release-6.1 and release-6.2, including build, check,
test, and integration pipelines.
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Pipeline script simplification:

* Removed redundant `script` blocks and duplicate calls to
`prow.setPRDescription(REFS)` from inside the `net-tool` container in
all affected pipeline scripts, reducing complexity and potential for
errors.
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These changes improve consistency and reliability in how PR metadata is
set for CI jobs.
 (commit: 4052ab8)