[bbejeck]

Fix to avoid flakiness in verifiable producer system test. The test lists running processes and greps to find the VerifiableProducer one, but wasn't providing an specific pattern to grep (so flaky if there were more than one process containing the default grep pattern "kafka")

Fix by passing a "proc_grep_string" to filter when looking for the VerifiableProducer process.

All test pass successfully after the change.

Reviewers: PoAn Yang payang@apache.org, Andrew Schofield aschofield@confluent.io

System test run (local)

 RunnerClient: kafkatest.sanity_checks.test_verifiable_producer.TestVerifiableProducer.test_simple_run.producer_version=dev.security_protocol=SSL.metadata_quorum=ISOLATED_KRAFT: Data: None
================================================================================
SESSION REPORT (ALL TESTS)
ducktape version: 0.12.0
session_id:       2026-03-05--001
run time:         9 minutes 26.513 seconds
tests run:        21
passed:           21
flaky:            0
failed:           0
ignored:          0
================================================================================