Follow-up from "macos: use GAUDI_LIBRARY_PATH instead of LD_..."

The following discussion from !1452 (merged) should be addressed:

• @clemenci started a discussion: (+2 comments)

  I was thinking that we may use the same logic on all platforms, may be calling the environment variable GAUDI_PLUGIN_PATH. If the library cannot be found in one of the directories listed there, then we can fallback on LD_LIBRARY_PATH.