chore(secretops): tweak configs relating to agenix-rekey stuff
In a nutshell:
- Add the `agenix-rekey` NixOS package overlay for system-wide access w/o
needing to "nix shell" first
- Add age and rage to the list of packages installed on all machines
- Minor tweak at agenix.nix for conditional enablement
Signed-off-by: Andrei Jiroh Halili <ajhalili2006@andreijiroh.dev>