beam_PostCommit_Py_ValCont should run with dataflow_worker_jar

[damccorm]

For the first failure, please refer to https://builds.apache.org/job/beam_PostCommit_Py_ValCont/5172/#showFailuresLink

Imported from Jira BEAM-9137. Original Jira may contain additional context.
Reported by: boyuanz.
Subtask of issue #19900