I agree with @rwcarlsen.

> I've implemented the environment *_PATH variable to operate exactly like the *nix PATH variable

That is a separate issue.  For packages (model collections, libraries, whatever) we will have `CYCLUS_PATH` variable that acts like PATH and is used to search known install locations. The purpose `CYCLUS_RNG_PATH` is to short-circuit that infrastructure. You should probably even leave off the `_PATH` here and just call it `CYCLUS_RNG` to avoid confusion.