Changes

opt(pingcap/tidb): set PR description earlier in all pipelines (#3679)

This pull request refactors how the PR description is set across
multiple Jenkins pipeline scripts for the `pingcap/tidb` repository.
Instead of calling `prow.setPRDescription(REFS)` inside a container
step, the call is now made immediately after parsing the PR references
at the start of each pipeline. This change improves consistency and
ensures the PR description is set earlier in the pipeline execution.

**Refactoring of PR description setting:**

* Moved the `prow.setPRDescription(REFS)` call to immediately after the
`REFS` variable is initialized in all affected pipeline scripts
[[1]](diffhunk://#diff-aca7042b465f87b631581ba98f2c7e07a0e746e4b280fc3a20958c8844aa15c5R11)
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[[20]](diffhunk://#diff-e645c7bc98dba354105974898bab636ce65b1af9013b269f50c3bc3d7a152f2bR11).

* Removed redundant calls to `prow.setPRDescription(REFS)` from within
the `net-tool` container script block in all affected pipeline scripts,
simplifying the container steps
[[1]](diffhunk://#diff-330efc710fae301d014390d100e1ec4c12e0ca09a6f6118bfdfc481706b5cc1cL39-L41)
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[[10]](diffhunk://#diff-43ec52ea3a8dfba70c88e7dd15c028b11a23f97755b697a2fe3d9f52231e1e81L39-L41).

This change streamlines the pipeline code, reduces duplication, and
ensures the PR description is set at a consistent and logical point in
the pipeline execution.
 (commit: 048524d)