Note on building in windows given multiple Python installations

[coarsedamavand]

Added Note under the 'build the code in the workspace' section: https://docs.ros.org/en/rolling/Installation/Alternatives/Windows-Development-Setup.html#build-the-code-in-the-workspace

Informs users of cmake-args useful in setting specific Python directories for the windows build on a system with multiple Python installations.

[clalancette]

Can you try with --cmake-args -DPython3_EXECUTABLE=C:\path\to\exe? That is what we are going for elsewhere, and I think that is enough without all of the extra settings.

[coarsedamavand]

Can you try with --cmake-args -DPython3_EXECUTABLE=C:\path\to\exe? That is what we are going for elsewhere, and I think that is enough without all of the extra settings.

Not sure what's changed since I ran this last, but neither my commit nor your suggestion complete when building rosidl_generator_py, with the wrong python version being found at line 22 of rosidl_generator_py_generate_interfaces.cmake: find_package(Python3 REQUIRED COMPONENTS Interpreter Development NumPy)

However, building with the following cmake args DOES complete successfully:
-DPYTHON_EXECUTABLE=C:/Python38/python.exe
-DPython3_ROOT_DIR=C:/Python38
-DPython3_FIND_STRATEGY=LOCATION
-DPython3_FIND_REGISTRY=NEVER
I'll check if all 4 of these are necessary.

[coarsedamavand]

I did indeed need all 4 cmake args. I'm running on Windows 10, with:
Python 3.8.3 installed from chocolatey in the default location, C:/Python38
And Python 3.9.12 from the web via exe installer, and opted to add it to path during installation.

I will update the documentation accordingly in another commit