fix hierarchical mns w.r.t. MPI installed with a dummy toolchain

enhanced version of #983 by @kcgthb (which should have been opened targeting `develop`
There is a very good reason: there's a difference between installing a module for `icc` itself, which should include the proper extensions to `$MODULEPATH` via `module use` statements, and installing modules on top of a toolchain that includes `icc`.

In this case, we're checking whether `icc` itself is being installed. Using `det_toolchain_compilers_name_version` doesn't make sense, since the `dummy` toolchain is used when installing `icc`.