UpdateContainer: pass suffix instead of DecoderPlugin

Instead of using the first DecoderPlugin that supports the suffix, use
the first one that actually implements the "container_scan" method.

src/UpdateContainer.hxx

@@ -31,6 +31,6 @@ bool
 update_container_file(Directory &directory,
 		      const char *name,
 		      const struct stat *st,
-		      const DecoderPlugin &plugin);
+		      const char *suffix);
 
 #endif