Fix validation of the post-training, notebook DAGs

[ooops678]

Description

Fix validation of post-training for notebook not working as expected.
Before this fix, the validation always passes because grep will find the source code that contains the success message but we want the actual success message in the output.

Tests

• Airflow Test Results: https://f5eac1c8d1684611864003492b988589-dot-us-east1.composer.googleusercontent.com/home?tags=orbax

Airflow/Composer

• GCP Composer name: camilo-orbax-dev (under GCP project: cloud-ml-auto-solutions)
• GCP Composer version: 2.13.1

Bucket with HNS (Hierarchical Name Space)

• Bucket Name: gs://mtc-automation-bucket

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run one-shot tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed.