Change build.sh to find C++ library by default

[vyasr]

Discussions with the team indicated a general preference for having build.sh default to finding the C++ library by default (and erroring if one is not found) rather than building a new copy of libraft when building raft Python.

[cjnolet]

rather than building a new copy of libcuspatial when building cuspatial Python.

Should this say libraft and raft Python?

[vyasr]

rather than building a new copy of libcuspatial when building cuspatial Python.

Should this say libraft and raft Python?

Nope. Copy-pasting is always correct ;)
Fixed, thanks for the catch.

[cjnolet]

@gpucibot merge