Commit 89fee1006c ("nixos/redis: clean up
option types") broke nixos evaluation:
error: attempt to call something which is not a function but a set, at .../nixpkgs/nixos/modules/services/databases/redis.nix:111:28
Fix it.
Reported by Oliver Charles (thanks!).
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
