Support virtual environment for Python

LLGo uses pkg-config to look up python lib path, however, when user is in virtual environment, the pkg-config result may not be acurate.

Example:
There's multi python version in my laptop.
Main: Python 3.13
Conda: Python 3.12.

The pkg-config result:

(base) matrix % pkg-config --libs  python3-embed
-L/opt/homebrew/opt/python@3.13/Frameworks/Python.framework/Versions/3.13/lib 

After i switch environment:

(llgo)  matrix % pkg-config --libs  python3-embed 
-L/opt/homebrew/opt/python@3.13/Frameworks/Python.framework/Versions/3.13/lib

It remains the same.

Even though some python virtual enviroment like Conda provides PC file for pkg-config, we cannot confirm users' virtual enviroment installation path, thus, it's hard to get the real python lib path from pkg-config.

The easy solution is that using python command to get it, like that:

python -c "import os; print('-L%s' % os.path.dirname(os.path.dirname(os.__file__)));"

Start by locating the Python library path lookup that currently invokes pkg-config for python3-embed. Compare its result with the active environment's python command, then verify the lookup uses the selected virtual environment rather than the system installation when switching between Python versions.