fix importfeed --force skip behavior reversion

importfeed --force: Don't treat it as a failure when an already downloaded
file exists. (Fixes a behavior change introduced in 10.20230626.)

04ee6c4c6b caused the reversion. Inside a CommandPerform, stop causes it
to fail. Before that commit, it was inside a CommandStart, where stop
causes it to skip.

Command/ImportFeed.hs

@@ -412,7 +412,7 @@ runDownload todownload url extension cache cv getter = do
 	case dest of
 		Nothing -> do
 			recordsuccess
-			stop
+			next $ return True
 		Just f -> getter (toRawFilePath f) >>= \case
 			Just ks
 				-- Download problem.