AppVeyor: update AppImage cache

only `libsndfile.a` instead of `libsndfile.so` was found causing the build to fail. Maybe the cache was somehow damaged?

.appveyor.yml

@@ -258,7 +258,7 @@ for:
          exit 0
       fi
 
-      cache_tag=usr_cache_appimage_fresh # this can be modified to rebuild deps
+      cache_tag=usr_cache_appimage_fresh_2 # this can be modified to rebuild deps
 
       ## Since we use a different Linux image we, unfortunately, have
       ## to use a separate cache.