remotes: FetchByDigest: propagate media type from config to desc

A media type string passed via `WithMediaType()` was not propagated
to a descriptor returned by `FetchByDigest()`.

Follow-up to PR 8744

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

cmd/ctr/commands/content/content.go

@@ -491,10 +491,11 @@ var (
 				if err != nil {
 					return err
 				}
-				rc, _, err := fetcherByDigest.FetchByDigest(ctx, dgst, remotes.WithMediaType(context.String("media-type")))
+				rc, desc, err := fetcherByDigest.FetchByDigest(ctx, dgst, remotes.WithMediaType(context.String("media-type")))
 				if err != nil {
 					return err
 				}
+				log.G(ctx).Debugf("desc=%+v", desc)
 				_, err = io.Copy(os.Stdout, rc)
 				rc.Close()
 				if err != nil {