[SPARK-50394][PYTHON][INFRA] Reduce parallelism in Pure Python library builds

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

This PR proposes to decrease parallelism in Pure Python library builds

### Why are the changes needed?

In order to make the tests more robust:

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

Now they fail because of OOM.

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

No, test-only

### How was this patch tested?

Will monitor the build:

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

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

No.

Closes #48932 from HyukjinKwon/reduce-parallelism.

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

Author: HyukjinKwon

.github/workflows/build_python_connect.yml

@@ -93,7 +93,7 @@ jobs:
           # Several tests related to catalog requires to run them sequencially, e.g., writing a table in a listener.
           ./python/run-tests --parallelism=1 --python-executables=python3 --modules pyspark-connect,pyspark-ml-connect
           # None of tests are dependent on each other in Pandas API on Spark so run them in parallel
-          ./python/run-tests --parallelism=4 --python-executables=python3 --modules pyspark-pandas-connect-part0,pyspark-pandas-connect-part1,pyspark-pandas-connect-part2,pyspark-pandas-connect-part3
+          ./python/run-tests --parallelism=2 --python-executables=python3 --modules pyspark-pandas-connect-part0,pyspark-pandas-connect-part1,pyspark-pandas-connect-part2,pyspark-pandas-connect-part3
 
           # Stop Spark Connect server.
           ./sbin/stop-connect-server.sh

.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=4 --python-executables=python3 --modules pyspark-pandas-connect,pyspark-pandas-slow-connect
+          ./python/run-tests --parallelism=2 --python-executables=python3 --modules pyspark-pandas-connect,pyspark-pandas-slow-connect
       - name: Upload test results to report
         if: always()
         uses: actions/upload-artifact@v4