python3 command update $PATH.

https://github.com/msys2/MINGW-packages/blob/master/mingw-w64-python3/0565-mingw-add-ModuleFileName-dir-to-PATH.patch#L28

Vim can use python interface called if_python using libpython3.7m.dll. When using Vim with if_python, Py_Initialize update $PATH environment variable. So when using another python environment like miniconda and open python file in Vim, :!python command use mingw's python.exe instead of conda's one.