Add special cases for Guix /gnu/store, mirroring the ones for NixOS /nix

This is not sufficient to make pressure-vessel fully operational on Guix, which will require help from the Guix side (notably ensuring that there is a usable ld.so.cache that we can use to enumerate available libraries), but it's a step towards that.

Part of https://github.com/ValveSoftware/steam-runtime/issues/478

/cc @denittis