If a cache index file is loaded which is older than a blacklist
or trusted.libraries file or contains an older variant of
file version 603 or 602 then we attempt to update the index file
"in place". We did not distinguish between sys vs. user caches.
This is wrong and we should only rewrite "in place" user caches unless we
are in system cache mode in order to avoid inefficient error handling and
attempted cache invalidations when the system cache is read-only.