Do not prepend source_root to build_dir

Prepending source_root is not only incorrect here but it talso breaks the build when `--build-lib` specifies an absolute path.

If `build_lib` specifies an absolute path, the path should be used as-is. When it is relative, it should be treated as relative to the current directory and not `source_root`. In this case, using the path as-is works as well.