flake: add package for cachix deploy spec

flake.nix

@@ -72,6 +72,10 @@
             };
 
             packages = pkgs.lib.optionalAttrs defaultPlatform {
+              cachix-deploy-spec = pkgs.writeText "cachix-deploy.json" (builtins.toJSON {
+                agents =
+                  pkgs.lib.mapAttrs (_: darwin: builtins.unsafeDiscardStringContext darwin.config.system.build.toplevel) self.darwinConfigurations;
+              });
               pages = pkgs.runCommand "pages"
                 {
                   buildInputs = [ config.devShells.mkdocs.nativeBuildInputs ];