Changes

fix(pingcap/tidb): add `-next-gen` flag to pluginpkg commands for plugins (#3597)

This pull request updates the pipeline configuration to use the
next-generation plugin packaging approach for compiling Go plugins. The
changes ensure that the `pluginpkg` command includes the `-next-gen`
flag when processing plugins.

Pipeline configuration updates:

*
[`pipelines/pingcap/tidb/latest/pull_build_next_gen/pipeline.groovy`](diffhunk://#diff-5d07f8a0316f7735c0722f8e9c4a63832c0fdef8f6ce42ee56dac322b85304c1L99-R103):
Updated the `pluginpkg` command to include the `-next-gen` flag for both
the `audit` and `whitelist` Go plugins, reflecting a shift to the
next-generation plugin packaging method.
 (commit: 5afecb3)