add run-vm-test-offline package for offline VM testing

This package allows running NixOS VM tests in an offline environment
using network namespace isolation. It builds the test driver and runs
it with unshare to ensure no network access.

checks/flake-module.nix

@@ -112,6 +112,9 @@ in
             cp ${../flake.lock} $out/flake.lock
           '';
         };
+      packages = lib.optionalAttrs (pkgs.stdenv.isLinux) {
+        run-vm-test-offline = pkgs.callPackage ../pkgs/run-vm-test-offline { };
+      };
       legacyPackages = {
         nixosTests =
           let