Use -DINSTALL_PROPRIETARY:BOOL=ON when calling develop.py from build.sh

Required because indra/cmake/Variables.cmake uses indra/Server.cmake as a flag to indicate this is an internal build, and set INSTALL_PROPRIETARY to download and install pre-built LLKDU, FMOD and Quicktime.

We need a better way of doing that, but this should get things going for now.