Changes

feature(pingcap/tidb): enable job `pull_integration_ddl_test_next_gen` job (#3660)

This pull request focuses on restructuring and optimizing the CI/CD
pipelines for integration and MySQL tests, as well as making small
configuration adjustments. The most significant changes include removing
unnecessary debug stages, improving caching and artifact handling, and
ensuring better script reliability.

### Pipeline restructuring and optimization:

* Removed the `Debug info` stage from the
`pull_integration_ddl_test_next_gen` pipeline, which included commands
for printing environment variables, debugging commands, and network
checks. This simplifies the pipeline by removing redundant steps.
(`pipelines/pingcap/tidb/latest/pull_integration_ddl_test_next_gen/pipeline.groovy`,
[pipelines/pingcap/tidb/latest/pull_integration_ddl_test_next_gen/pipeline.groovyL32-L48](diffhunk://#diff-30290e50879c98b2b0d4bd9095242cb7de09be03866928f0674f312cd397b44eL32-L48))
* Improved the `Prepare` stage in the
`pull_integration_ddl_test_next_gen` pipeline by introducing caching for
`tidb` and `tidb-test` binaries, consolidating artifact downloads, and
removing redundant container usage. This enhances efficiency and reduces
build times.
(`pipelines/pingcap/tidb/latest/pull_integration_ddl_test_next_gen/pipeline.groovy`,
[pipelines/pingcap/tidb/latest/pull_integration_ddl_test_next_gen/pipeline.groovyL74-R80](diffhunk://#diff-30290e50879c98b2b0d4bd9095242cb7de09be03866928f0674f312cd397b44eL74-R80))

### Script reliability improvement:

* Added `set -euo pipefail` to the MySQL test script to ensure stricter
error handling, improving script reliability and maintainability.
(`pipelines/pingcap/tidb/latest/pull_mysql_test_next_gen/pipeline.groovy`,
[pipelines/pingcap/tidb/latest/pull_mysql_test_next_gen/pipeline.groovyR132-R133](diffhunk://#diff-bace5d09f771329c3a3e1f6568129f82595f20ebc2866a7963be3053f86037fdR132-R133))

### Configuration adjustment:

* Enabled the `skip_if_only_changed` field in the
`pull_integration_ddl_test_next_gen` presubmit configuration, ensuring
that the job is skipped for irrelevant changes.
(`prow-jobs/pingcap/tidb/latest-presubmits-next-gen.yaml`,
[prow-jobs/pingcap/tidb/latest-presubmits-next-gen.yamlL95-R95](diffhunk://#diff-6d136e7cf0b5751935f337e8b7065f23f4f547f7f85072637dbab6476832439eL95-R95))
 (commit: d6c13d2)