modules: secrets: move wireguard keys

This is a bit special, as some of the keys do not belong to NixOS hosts,
so store those in the module itself, and into host-specific directories
for the keys that are NixOS hosts.

hosts/nixos/aramis/secrets/secrets.nix

@@ -9,5 +9,5 @@ let
   ];
 in
 {
-  # Add secrets here
+  "wireguard/private-key.age".publicKeys = all;
 }