nixos/redis: enforce owner/perms on /var/lib/redis (#18046)

Previously it was only set once, now it is enforced on each start-up of
redis.service. Also set _ownership_ recursively, so that the
/var/lib/redis/dump.rdb file is guaranteed to be accessible by the
currently configured redis user.

Fixes issue #9687, where redis wouldn't start because /var/lib/redis had
wrong owner.