[SPARK-32419][PYTHON][BUILD] Avoid using subshell for Conda env (de)activation in pip packaging test

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

This PR proposes to avoid using subshell when it activates Conda environment. Looks like it ends up with activating the env within the subshell even if you use `conda` command.

### Why are the changes needed?

If you take a close look for GitHub Actions log:

```
 Installing dist into virtual env
Processing ./python/dist/pyspark-3.1.0.dev0.tar.gz
Collecting py4j==0.10.9
 Downloading py4j-0.10.9-py2.py3-none-any.whl (198 kB)
Using legacy setup.py install for pyspark, since package 'wheel' is not installed.
Installing collected packages: py4j, pyspark
 Running setup.py install for pyspark: started
 Running setup.py install for pyspark: finished with status 'done'
Successfully installed py4j-0.10.9 pyspark-3.1.0.dev0

...

Installing dist into virtual env
Obtaining file:///home/runner/work/spark/spark/python
Collecting py4j==0.10.9
 Downloading py4j-0.10.9-py2.py3-none-any.whl (198 kB)
Installing collected packages: py4j, pyspark
 Attempting uninstall: py4j
 Found existing installation: py4j 0.10.9
 Uninstalling py4j-0.10.9:
 Successfully uninstalled py4j-0.10.9
 Attempting uninstall: pyspark
 Found existing installation: pyspark 3.1.0.dev0
 Uninstalling pyspark-3.1.0.dev0:
 Successfully uninstalled pyspark-3.1.0.dev0
 Running setup.py develop for pyspark
Successfully installed py4j-0.10.9 pyspark
```

It looks not properly using Conda as it removes the previously installed one when it reinstalls again.
We should ideally test it with Conda environment as it's intended.

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

No, dev-only.

### How was this patch tested?

GitHub Actions will test. I also manually tested in my local.

Closes apache#29212 from HyukjinKwon/SPARK-32419.

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

Author: HyukjinKwon

dev/run-pip-tests

@@ -86,7 +86,7 @@ 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
-      source activate "$VIRTUALENV_PATH" || (echo "Falling back to 'conda activate'" && conda activate "$VIRTUALENV_PATH")
+      source activate "$VIRTUALENV_PATH" || conda activate "$VIRTUALENV_PATH"
     else
       mkdir -p "$VIRTUALENV_PATH"
       virtualenv --python=$python "$VIRTUALENV_PATH"
@@ -129,7 +129,7 @@ for python in "${PYTHON_EXECS[@]}"; do
 
     # conda / virtualenv environments need to be deactivated differently
     if [ -n "$USE_CONDA" ]; then
-      source deactivate || (echo "Falling back to 'conda deactivate'" && conda deactivate)
+      source deactivate || conda deactivate
     else
       deactivate
     fi