purism-librem5: use gnss-share with geoclue

This fixes gnss out of the box on the librem 5, by configuring geoclue to use gnss-share as a nmea source.

I went this route, of overriding a .service file that invokes geoclue with a command line option, because forking this .service file is much nicer than forking geoclue.conf. The conf file will likely change over time to add new app permissions or whatever, and it's unlikely (hopefull!) that this .service file will change much upstream...