Remove CARLA build system dependency with py.exe in Windows

[xavisolesoft]

Problem

Some build scripts use py command instead python in Windows. This causes environments like anaconda to crash building CARLA PythonAPI as py command is just probided in the official python installer.

Solution

Replace all py invocations in CARLA build system by the apropiete python command.

Example of py command used in our build system: https://github.com/carla-simulator/carla/blob/04a46d6158fad3b3bf9a43e4635a35b61e40c58b/Util/BuildTools/BuildPythonAPI.bat#L112

Note that next documentation line should be updated: https://github.com/carla-simulator/carla/blob/04a46d6158fad3b3bf9a43e4635a35b61e40c58b/Docs/build_faq.md?plain=1#L368