flake: add `host-pkgs.<host>.<pkg>` output

this builds packages precisely as they are defined by the given host.
significant for testing whether a cross-compiled host builds things
correctly, for example.

flake.nix

@@ -111,6 +111,8 @@
       #   - `nixos-rebuild --flake './#<host>' switch`
       imgs = builtins.mapAttrs (_: host-dfn: host-dfn.config.system.build.img) self.nixosConfigurations;
 
+      host-pkgs = builtins.mapAttrs (_: host-dfn: host-dfn.config.system.build.pkgs) self.nixosConfigurations;
+
       overlays = rec {
         default = pkgs;
         pkgs = import ./overlays/pkgs.nix;

modules/programs.nix

@@ -9,6 +9,7 @@ let
     mapAttrsToList
     mkDefault
     mkIf
+    mkMerge
     mkOption
     optional
     optionalAttrs
@@ -122,6 +123,11 @@ in
         users.users = f.users.users;
         sane.users = f.sane.users;
       };
-    in
-      take (sane-lib.mkTypedMerge take configs);
+    in mkMerge [
+      (take (sane-lib.mkTypedMerge take configs))
+      {
+        # expose the pkgs -- as available to the system -- as a build target.
+        system.build.pkgs = pkgs;
+      }
+    ];
 }