Changes

chore(pingcap/tidb): disable auto trigger `pull_unit_test_next_gen` job (#3682)

This pull request makes a small change to the presubmit job
configuration for `pingcap/tidb` in
`prow-jobs/pingcap/tidb/latest-presubmits-next-gen.yaml`. The change
comments out the `skip_if_only_changed` field for the
`pull_unit_test_next_gen` job, which means this job will now always run
regardless of which files are changed.
 (commit: 7f3d1ee)