util/disk_cache: add new driver_flags param to cache keys

util/disk_cache: add new driver_flags param to cache keys

This will be used for things such as adding driver specific environment
variables to the key. Allowing us to set environment vars that change
the shader and not have the driver ignore them if it finds existing
shaders in the cache.

Reviewed-by: Eduardo Lima Mitev <elima@igalia.com>