Parquet: accept existing python env

[KIwabuchi]

This PR makes YGM take a path to an existing Python virtual environment directory and use the location for finding/installing pyarrow.
This PR is also useful for specifying a specific Python executable.

/path/to/python3 -m venv $HOME/venv/my-venv
source $HOME/venv/my-venv/bin/activate
# If pyarrow is not installed, YGM's will do that during the CMake configuration.
pip install pyarrow==20.0.*
deactivate

# Build YGM
cd ygm//build
cmake ../ -DYGM_INSTALL_PARQUET=ON -DYGM_PYTHON_VENV_ROOT=$HOME/venv/my-venv