substitute: Stop using call-with-cached-connection in fetch-narinfos.

Instead, just pass open-connection-for-uri/maybe to http-multiple-get. This
code should be functionaly similar to the previous code. The eventual aim of
this is to make the connection caching not mandatory in fetch-narinfos.

* guix/scripts/substitute.scm (fetch-narinfos): Remove use of
call-with-cached-connection.