generate_mcpp.bat script seems to have stale paths

[siliconvoodoo]

@echo off
set DEVPATH=%1
set MCPP=%DEVPATH%\Gems\Atom\Asset\Shader\External\mcpp\2.7.2-az.1\lib\win_x64\mcpp.exe
set AZSLC=..\..\..\bin\win_x64\Release\azslc.exe
set DXC=%DEVPATH%\Gems\Atom\Asset\Shader\External\DirectXShaderCompiler\2020.08.07\bin\win_x64\Release\dxc.exe"

set AZSL=%2
set PREPROCESSED=%AZSL%.mcpp
set HLSL=%PREPROCESSED%.hlsl

rem %MCPP% %AZSL% > %PREPROCESSED%

%AZSLC% %PREPROCESSED% -o %HLSL%

rem %DXC% -help
rem %DXC% -T cs_6_2 %HLSL%
rem @echo on
rem %DXC% -T cs_6_2 main.azsl.mcpp.hlsl2.hlsl

If I understand correctly the changes that occurred for o3de release made that this test script cannot run from where it stands today.
It is in o3de-azslc\tests\Advanced\RespectEmitLine
I suspect that we need the introduction of an environment variable to be able to locate mcpp.exe which appears to be in o3de-packages\packages\mcpp-2.7.2_az.2-rev1-windows\mcpp\lib
(a python test script could also make the 2.7.2_.... part a non-necessary part with a regex like /mcpp-.*/)