fix: Update path of batch-predictor's main.py (#569)

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# Description

To maintain backward compatibility with images created prior to 0.41, we
will revert the merlin-batch-predictor's main.py location.

# Modifications
<!-- Summarize the key code changes. -->

Update batch-predictor's main.py location.

# Tests
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scenarios should be tested? Consider the backward compatibility of the
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- [ ] Deploying PyFunc models
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# Checklist
- [ ] Added PR label
- [ ] Added unit test, integration, and/or e2e tests
- [ ] Tested locally
- [ ] Updated documentation
- [ ] Update Swagger spec if the PR introduce API changes
- [ ] Regenerated Golang and Python client if the PR introduces API
changes

# Release Notes
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```release-note

```

config.yaml

@@ -36,7 +36,7 @@ ImageBuilderConfig:
     DockerfilePath: docker/app.Dockerfile
     BuildContextURI: git://github.com/gojek/merlin.git#refs/tags/v0.37.0
     BuildContextSubPath: python
-    MainAppPath: /home/spark/main.py
+    MainAppPath: /home/spark/merlin-spark-app/main.py
   BuildNamespace: mlp
   DockerRegistry: ghcr.io/caraml-dev
   BuildTimeout: 10m

python/batch-predictor/docker/base.Dockerfile

@@ -81,6 +81,6 @@ RUN wget --quiet https://github.com/conda-forge/miniforge/releases/download/${MI
     $CONDA_DIR/bin/conda clean -afy && \
     echo "source $CONDA_DIR/etc/profile.d/conda.sh" >> $HOME/.bashrc
 
-COPY batch-predictor/docker/main.py ${HOME}/main.py
+COPY batch-predictor/docker/main.py ${HOME}/merlin-spark-app/main.py
 COPY batch-predictor/docker/merlin_entrypoint.sh /usr/bin/merlin_entrypoint.sh
 COPY batch-predictor/docker/process_conda_env.sh /usr/bin/process_conda_env.sh