Don't use cached items if --no-cache is specified and proxy is used
Currently apk would use cached in proxy copy of item it is fetching from repository because it does not indicate that it would prefer to use newest version. It would be useful if when --no-cache or other new option is used, apk would add "Cache-Control" HTTP header with value "no-cache" to ensure that newest copy of index and/or package is being downloaded.
libfetch: add option to set "Cache-Control: no-cache"
(cherry picked from commit f90af35e9c563bd4f865d8d47a7ae357191494db)