Currently in NixOS configuration using agenix secrets there is no
build time validation of secret paths - things fail at runtime (system
activation).
To prevent that, this CL makes the secrets part of the tree based on
the same configuration file used by agenix itself.
This guards against:
* agenix secrets.nix definition for a non-existent file
* age.secrets value in a NixOS config for a non-existent secret
Change-Id: I5b191dcbd5b2522566ff7c38f8a988bbf7679364
3a410a78df