Changes

opt(pipelines/pingcap/ticdc/latest): save the main flow pod for classic CI jobs (#4106)

This pull request refactors several TiCDC integration test pipelines to
streamline the build and preparation stages. The main improvements
include merging the checkout and preparation steps into a single stage,
introducing per-stage agents, and improving caching for downstream test
stages. These changes make the pipelines more efficient and easier to
maintain.

**Pipeline structure improvements:**

* Merged the "Checkout" and "Prepare" stages into a single `Checkout &
Prepare` stage in all affected pipelines, reducing redundancy and
improving clarity.
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* Changed the pipeline-level `agent` from a Kubernetes agent to `agent
none`, and moved the Kubernetes agent definition to the new `Checkout &
Prepare` stage, allowing for more flexible agent assignment in other
stages.
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**Build and caching enhancements:**

* Added comments and structure to clarify the steps for checking out
code, building binaries, and downloading additional required binaries.
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* Improved binary caching by ensuring built artifacts are cached for use
in downstream test stages, reducing redundant builds and speeding up
pipeline execution.
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* Explicitly added a step for downloading other binaries needed for the
tests, improving clarity and reproducibility.
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These changes collectively improve pipeline maintainability, execution
speed, and clarity for future contributors.

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
 (commit: 43a77d4)