chef: make cached artifact uri platform independent

Cached artifact uri was previously constructed manually using a string
representation of the artifact path. This change ensures that the uri 
is generated by pathlib instead.

Resolves: #2744

Co-authored-by: Fredrik Averpil <fredrik.averpil@ericsson.com>
Co-authored-by: Arun Babu Neelicattu <arun.neelicattu@gmail.com>