Various fixes for build.sh

[vyasr]

This PR includes the following fixes for build.sh:

• The parallel argument is for the build tool (make/ninja) and not CMake or setup.py, so it needs to be passed after a second --.
• pylibraft requires the distances component of libraft to be compiled. If pylibraft is passed to build.sh, we need to force building libraft with the distance component
• build.sh doesn't install by default, so the CMAKE_PREFIX_PATH that the Python build points to needs to also include the build directory in addition to the install directory. I think giving the build directory precedence makes more sense, but that could be changed if necessary.

[teju85]

I confirm that this PR fixes the issues I was facing: (Thanks @vyasr !)

1. parallel build not working
2. "distances" sources getting compiled twice (due to me having to set FIND_RAFT_CPP=OFF)

After this PR gets merged, if one wants reuse the compiled sources from cpp/src, then one needs to make sure NOT to pass the above FIND_RAFT_CPP=OFF option!

That said, there's still a minor hiccup: the python setup.py ... commands in build.sh somehow doesn't seem to honor setting up ccache through options like -DCMAKE_CUDA_COMPILER_LAUNCHER=ccache. For now, this is not an issue for me as I'll not be choosing the FIND_RAFT_CPP=OFF path.

[teju85]

@gpucibot merge