Handle UE versions with old bundled Python paths on Mac/Linux#4

Python has not always lived in the same place for Unreal. I've handled this on Windows. Mac/Linux need the same treatment. Most users can likely rely on Python already being installed on those platforms, but using the bundled version is still ideal. On Windows: - In 4.9-4.12 Python was: `$EngineDir\Source\ThirdParty\HTML5\emsdk\Win64\python\2.7.5.3_64bit\python.exe` - In 4.13-4.18 Python was: `$EngineDir\Extras\ThirdPartyNotUE\emsdk\Win64\python\2.7.5.3_64bit\python.exe` - In 4.19-4.21 Python was: `$EngineDir\Source\ThirdParty\Python\Win64\python.exe` - In 4.22-4.25 Python was: `$EngineDir\Binaries\ThirdParty\Python\Win64\python.exe` - In 4.26 and up Python is: `$EngineDir\Binaries\ThirdParty\Python3\Win64\python.exe` See Prebuild.ps1 for Windows implementation that needs to be ported to the Mac/Linux Prebuild.sh files.