`IDRIS_LIBRARY_PATH` should follow `PATH` convention

[raingloom]

I'm (still) trying to get Idris 2 compiling on Guix and ran into a weird error with guix environment --ad-hoc idris: Idris 1 can't find the base and prelude libraries when IDRIS_LIBRARY_PATH has more than one component, such as: /gnu/store/xbp84d145j61m7c70njxn5jkvb0590z8-profile/lib/idris:/home/raingloom/.guix-profile/lib/idris/

The solution is probably simple, I'll try to send a patch soon.