Changes

feat(tidbcloud/cloud-storage-engine): fix and enable nextgen real cluster test job (#3835)

This pull request makes several improvements to the CI pipeline for the
TiDB Cloud Storage Engine's dedicated integration tests. The main focus
is on enhancing reliability, efficiency, and security of the build and
test process. Key changes include improved Git checkout procedures,
optimized binary build and caching logic, and updates to job
configuration for better control and clarity.

**Pipeline reliability and efficiency improvements:**

* Switched from `prow.checkoutRefs` to `prow.checkoutPrivateRefs` with a
specified GitHub credentials ID (`GIT_CREDENTIALS_ID`) and a timeout,
improving security and reliability for private repo checkouts.
[[1]](diffhunk://#diff-be43e5ef9fd98036ec25c7720a1cd9c890b715506b7f2455f83db0ede1dba9aeR8)
[[2]](diffhunk://#diff-be43e5ef9fd98036ec25c7720a1cd9c890b715506b7f2455f83db0ede1dba9aeL39-R40)
* Enhanced the build process to check for existing binaries in the `bin`
directory before building, restoring from cache if present, and using
`mv` instead of `cp` to avoid duplicate files. This prevents unnecessary
rebuilds and speeds up the pipeline.
[[1]](diffhunk://#diff-be43e5ef9fd98036ec25c7720a1cd9c890b715506b7f2455f83db0ede1dba9aeL61-R67)
[[2]](diffhunk://#diff-be43e5ef9fd98036ec25c7720a1cd9c890b715506b7f2455f83db0ede1dba9aeL73-R80)
* Improved the script for downloading and copying peer component
binaries, adding error handling and consistent use of `cp` for clarity
and reliability.

**Pipeline configuration and security:**

* Added a privileged security context to the `golang` container in
`test-pod.yaml`, which may be required for certain build or test
operations.
* Updated the presubmit job configuration to enable
`skip_if_only_changed`, changed the context to
`non-block/pull-integration-realcluster-test-next-gen`, and removed the
`always_run` flag, making the job optional and non-blocking until tests
are stabilized.

**Other changes:**

* Removed the explicit timeout option from the 'Test' stage, possibly to
rely on default or external timeout management.

---
Close #3804
 (commit: 0e74b0f)