two source_sampling.so files in docker image

[billingsley-john]

When the parametric_plasma_source is compiled a source_sampling.so file is created in the /build directory. However, another source_sampling.so file is also created in the parametric_plasma_source directory (I assume when the plasma source is pip installed). SOURCE_SAMPLING_PATH points to the incorrect source_sampling.so file - it should point to /build/source_sampling.so but it points to /parametric_plasma_source/source_sampling.so