pull: Don't save into cache passed in GByte summaries

pull: Don't save into cache passed in GByte summaries

The cache shouldn't be affected by the user passing in some other
summary as it may not be the "official one".

I ran into this in flatpak where the passed summary was correct, but
the re-saving of the cache updated the mtime of the cached file which
led to later http If-Modified-Since calls failing to update.