substitute: Change connection cache handling in process-substitution.

Just pass open-connection-for-uri/maybe to http-fetch, this removes the need
for with-cached-connection and passing the port in.

* guix/scripts/substitute.scm (fetch): Don't take a port as an argument, and
pass open-connection-for-uri/maybe to http-fetch.
(process-substitution): Don't call fetch with with-cached-connection.