Missing rpath to stubs subdir

When building openfoam with scotch but without kahip and metis, I end up with a broken executable:

$ ./platforms/linux64IcxDPInt32-spack/bin/decomposePar
./platforms/linux64IcxDPInt32-spack/bin/decomposePar: error while loading shared libraries: libkahipDecomp.so: cannot open shared object file: No such file or directory

because the stub libraries of openfoam live in

./platforms/linux64IcxDPInt32-spack/lib/dummy/

which is not part of the RPATH, only the lib dir is.