Document how to install `python-suitesparse-graphblas` with pre-installed SuiteSparse:GraphBLAS

[eriknw]

Some users may prefer to install SuiteSparse:GraphBLAS on their own, so we should document how to install python-suitesparse-graphblas from the source distribution (sdist) from PyPI or the developer version from the repository.

[alugowski]

I believe this was actually impossible? The build would look in sys.prefix on unix and C:/GraphBLAS on Windows. I made a PR to make this selectable with an environment variable, the same variable you have to set when using FindGraphBLAS with CMake.