radv: add debug option to turn off in memory cache

radv: add debug option to turn off in memory cache

This can be usefull for debugging the on disk cache, but is also
useful in the following patch for secure compiles which will be
used to compile huge pipeline collections. These pipeline
collections can be multiple GBs and the in memory cache grows to
multiple GBs very quickly when they are compiled so we want to
be able to turn off the in memory cache.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>