Optionally also give the store path

[Necoro]

When packaging something, it is always a good idea to look at the contents of the fetched asset. As nurl already downloads and unpacks the asset into the store, it would be great to have the store path output as well. To not break scripting, it probably should be behind an option switch.

Currently, I always execute the nix flake prefetch command as printed by nurl, as it contains the store. But this feels superfluous.