hosts/hydra-1: Sign nix cache entries with multiple keys

flake/overlay.nix

@@ -1,6 +1,7 @@
 { self
 , bij
 , chaosevents
+, harmonia
 , ssh-to-age
 , ...
 }@inputs:
@@ -9,6 +10,8 @@ final: prev: {
     bij;
   inherit (chaosevents.packages.${final.system})
     chaosevents;
+  inherit (harmonia.packages.${final.system})
+    harmonia;
   inherit (ssh-to-age.packages.${final.system})
     ssh-to-age;
 }