[SPARK-32363][PYTHON][BUILD] Fix flakiness in pip package testing in Jenkins

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

This PR proposes:

- Don't use `--user` in pip packaging test
- Pull `source` out of the subshell, and place it first.
- Exclude user sitepackages in Python path during pip installation test

to address the flakiness of the pip packaging test in Jenkins.

(I think) #29116 caused this flakiness given my observation in the Jenkins log. I had to work around by specifying `--user` but it turned out that it does not properly work in old Conda on Jenkins for some reasons. Therefore, reverting this change back.

(I think) the installation at user site-packages affects other environments created by Conda in the old Conda version that Jenkins has. Seems it fails to isolate the environments for some reasons. So, it excludes user sitepackages in the Python path during the test.

In addition, #29116 also added some fallback logics of `conda (de)activate` and `source (de)activate` because Conda prefers to use `conda (de)activate` now per the official documentation and `source (de)activate` doesn't work for some reasons in certain environments (see also conda/conda#7980). The problem was that `source` loads things to the current shell so does not affect the current shell. Therefore, this PR pulls `source` out of the subshell.

Disclaimer: I made the analysis purely based on Jenkins machine's log in this PR. It may have a different reason I missed during my observation.

### Why are the changes needed?

To make the build and tests pass in Jenkins.

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

No, dev-only.

### How was this patch tested?

Jenkins tests should test it out.

Closes #29117 from HyukjinKwon/debug-conda.

Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

Author: HyukjinKwon

dev/run-pip-tests

@@ -63,11 +63,15 @@ fi
 PYSPARK_VERSION=$(python3 -c "exec(open('python/pyspark/version.py').read());print(__version__)")
 PYSPARK_DIST="$FWDIR/python/dist/pyspark-$PYSPARK_VERSION.tar.gz"
 # The pip install options we use for all the pip commands
-PIP_OPTIONS="--user --upgrade --no-cache-dir --force-reinstall "
+PIP_OPTIONS="--upgrade --no-cache-dir --force-reinstall"
 # Test both regular user and edit/dev install modes.
 PIP_COMMANDS=("pip install $PIP_OPTIONS $PYSPARK_DIST"
 	      "pip install $PIP_OPTIONS -e python/")
 
+# Jenkins has PySpark installed under user sitepackages shared for some reasons.
+# In this test, explicitly exclude user sitepackages to prevent side effects
+export PYTHONNOUSERSITE=1
+
 for python in "${PYTHON_EXECS[@]}"; do
   for install_command in "${PIP_COMMANDS[@]}"; do
     echo "Testing pip installation with python $python"
@@ -81,7 +85,7 @@ for python in "${PYTHON_EXECS[@]}"; do
         source "$CONDA_PREFIX/etc/profile.d/conda.sh"
       fi
       conda create -y -p "$VIRTUALENV_PATH" python=$python numpy pandas pip setuptools
-      conda activate "$VIRTUALENV_PATH" || (echo "Falling back to 'source activate'" && source activate "$VIRTUALENV_PATH")
+      source activate "$VIRTUALENV_PATH" || (echo "Falling back to 'conda activate'" && conda activate "$VIRTUALENV_PATH")
     else
       mkdir -p "$VIRTUALENV_PATH"
       virtualenv --python=$python "$VIRTUALENV_PATH"
@@ -96,8 +100,6 @@ for python in "${PYTHON_EXECS[@]}"; do
     cd "$FWDIR"/python
     # Delete the egg info file if it exists, this can cache the setup file.
     rm -rf pyspark.egg-info || echo "No existing egg info file, skipping deletion"
-    # Also, delete the symbolic link if exists. It can be left over from the previous editable mode installation.
-    python3 -c "from distutils.sysconfig import get_python_lib; import os; f = os.path.join(get_python_lib(), 'pyspark.egg-link'); os.unlink(f) if os.path.isfile(f) else 0"
     python3 setup.py sdist
 
 
@@ -116,7 +118,6 @@ for python in "${PYTHON_EXECS[@]}"; do
     cd /
 
     echo "Run basic sanity check on pip installed version with spark-submit"
-    export PATH="$(python3 -m site --user-base)/bin:$PATH"
     spark-submit "$FWDIR"/dev/pip-sanity-check.py
     echo "Run basic sanity check with import based"
     python3 "$FWDIR"/dev/pip-sanity-check.py
@@ -127,7 +128,7 @@ for python in "${PYTHON_EXECS[@]}"; do
 
     # conda / virtualenv environments need to be deactivated differently
     if [ -n "$USE_CONDA" ]; then
-      conda deactivate || (echo "Falling back to 'source deactivate'" && source deactivate)
+      source deactivate || (echo "Falling back to 'conda deactivate'" && conda deactivate)
     else
       deactivate
     fi