feat: add a check for testing makeApps

flake.nix

···
     };
     checks =
       l.mapAttrs
-      (_: pkgs: {
+      (_: pkgs: let
+        testSystem = l.nixosSystem {
+          system = pkgs.system;
+          modules = l.attrValues inp.self.nixosModules;
+        };
+      in {
         firewall-hetzner-app = import ./firewall/hetzner/app.nix {
           inherit pkgs;
           taggedPorts = {
···
           };
           id = 1;
         };
+        test-system-app =
+          (inp.self.makeApps {
+            inherit pkgs; nixosSystem = testSystem;
+          }).run;
       })
       pkgsInstances;
     makeApps = import ./makeApps.nix;