engine: Add EKN_DISABLE_UPDATES envvar for debugging
engine: Add EKN_DISABLE_UPDATES envvar for debugging

When working on specific content sources, it's helpful to not try to
refetch new shards from the server, so add an envvar to help with this
use case.

endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib