Nar: Implement `mkLink` for `Arbitrary FileSystemObject`

`mkLink` in `hnix-store-core/tests/NarFormat.hs` is undefined with a TODO note to implement it. It seems the intention for it was to generate `SymLink`s of `FileSystemObject` in its `Arbitrary` instances.

This would be nice to have to have complete coverage of NAR encoding via `prop_narEncodingArbitrary` property.