guix-shared-cache-service-type: Fix unbound variable

Improve shared guix cache

- Add -o nonempty option to make sure the service can run even with
users that already have a cache
- Rename the config with -config suffix

guix-shared-cache-service-type: share only some of the folders

ruther: Add guix-shared-cache-service-type

Share .guix/cache between multiple users to cut down on size
and time taken to reconfigure initially.