geocode-nominatim: Allow the cache path to be adjusted
@pwithnall
Submitted by Philip Withnall Link to original bug (#777196)
Description
Split off from discussion in https://bugzilla.gnome.org/show_bug.cgi?id=756311#c160.
I still think it’s a good idea to allow these things to be configurable on a per-object basis rather than using global shared state ($XDG_CACHE_HOME) for them, but in the case of ensuring the unit tests don’t use a common cache, setting $XDG_CACHE_HOME works fine.