Sets the SUBJECTS_DIR variable in the system environment or
options("fs.subj_dir" = x)
set_fs_subj_dir(x = file.path(fs_dir(), "subjects"))
No return value, called for side effects (`SUBJECTS_DIR` environment variable set, and `fs.subj_dir` option set)
path to SUBJECTS_DIR defaults to file.path(fs_dir(), "subjects")