[SPARK-50394][PYTHON][INFRA][FOLLOW-UP] Reduce parallelism further in…

… Pure Python library builds

### What changes were proposed in this pull request?

This PR is a followup of #48932 that reduces parallelism further

### Why are the changes needed?

In order to make the tests more robust:

https://github.com/apache/spark/actions/workflows/build_python_connect35.yml

It fails because of OOM.

FWIW, https://github.com/apache/spark/actions/workflows/build_python_connect.yml is fixed by the original PR.

### Does this PR introduce _any_ user-facing change?

No, test-only.

### How was this patch tested?

Will monitor the build.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #48948 from HyukjinKwon/SPARK-50394-followup.

Authored-by: Hyukjin Kwon <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

.github/workflows/build_python_connect35.yml

@@ -98,7 +98,7 @@ jobs:
           # Run branch-3.5 tests
           ./python/run-tests --parallelism=1 --python-executables=python3 --modules pyspark-connect
           # None of tests are dependent on each other in Pandas API on Spark so run them in parallel
-          ./python/run-tests --parallelism=2 --python-executables=python3 --modules pyspark-pandas-connect,pyspark-pandas-slow-connect
+          ./python/run-tests --parallelism=1 --python-executables=python3 --modules pyspark-pandas-connect,pyspark-pandas-slow-connect
       - name: Upload test results to report
         if: always()
         uses: actions/upload-artifact@v4