Use python[w].exe adjacent to launcher

[konstin]

I'm not sure if that is the right thing for posy, i've built this for installing packages into a venv: When creating a launcher for let's say black, we put it in .venv\Scripts\black.exe. The python interpreter is adjacent in the same directory, either .venv\Scripts\python.exe or .venv\Scripts\pythonw.exe. This replaces the POSY_PYTHON environment variables.