nixos-container: force systemd-nspawn to use unified cgroups hierarchy

nixos/modules/virtualisation/nixos-containers.nix

···
         fi
       ''}
 
+      export SYSTEMD_NSPAWN_UNIFIED_HIERARCHY=1
+
       # Run systemd-nspawn without startup notification (we'll
       # wait for the container systemd to signal readiness)
       # Kill signal handling means systemd-nspawn will pass a system-halt signal

nixos/tests/all-tests.nix

···
   containers-reloadable = handleTest ./containers-reloadable.nix {};
   containers-restart_networking = handleTest ./containers-restart_networking.nix {};
   containers-tmpfs = handleTest ./containers-tmpfs.nix {};
+  containers-unified-hierarchy = handleTest ./containers-unified-hierarchy.nix {};
   convos = handleTest ./convos.nix {};
   corerad = handleTest ./corerad.nix {};
   coturn = handleTest ./coturn.nix {};

nixos/tests/containers-unified-hierarchy.nix

···
+import ./make-test-python.nix ({ pkgs, lib, ... }: {
+  name = "containers-unified-hierarchy";
+  meta = {
+    maintainers = with lib.maintainers; [ farnoy ];
+  };
+
+  nodes.machine = { ... }: {
+    containers = {
+      test-container = {
+        autoStart = true;
+        config = { };
+      };
+    };
+  };
+
+  testScript = ''
+    machine.wait_for_unit("default.target")
+
+    machine.succeed("echo 'stat -fc %T /sys/fs/cgroup/ | grep cgroup2fs' | nixos-container root-login test-container")
+  '';
+})

pkgs/tools/virtualization/nixos-container/default.nix

···
           containers-ip
           containers-tmpfs
           containers-ephemeral
+          containers-unified-hierarchy
           ;
       };
     };