This is also the directory where the model is built, which could be the
value of tempdir().
Examples
mod <- mrgsolve::house()
soloc(mod)
#> [1] "/private/var/folders/zv/v6tkdhrn1_bb1ndrc0c0j31w0000gp/T/RtmpbfDrGN/temp_libpath4b5b101c77d6/mrgsolve/libs"