nixos/tests/chromium: Switch to nodes.machine (vs. deprecated machine attribute)

The old attribute is deprecated:
trace: warning: In test `chromium-stable': The `machine' attribute in NixOS
tests (pkgs.nixosTest / make-test-pyton.nix / testing-python.nix / makeTest) is
deprecated. Please use the equivalent `nodes.machine'.

Note: This is only a refactoring.

nixos/tests/chromium.nix

@@ -45,13 +45,15 @@ mapAttrs (channel: chromiumPkg: makeTest {
 
   enableOCR = true;
 
-  machine.imports = [ ./common/user-account.nix ./common/x11.nix ];
-  machine.virtualisation.memorySize = 2047;
-  machine.test-support.displayManager.auto.user = user;
-  machine.environment = {
+  nodes.machine = { ... }: {
+    imports = [ ./common/user-account.nix ./common/x11.nix ];
+    virtualisation.memorySize = 2047;
+    test-support.displayManager.auto.user = user;
+    environment = {
       systemPackages = [ chromiumPkg ];
       variables."XAUTHORITY" = "/home/alice/.Xauthority";
     };
+  };
 
   testScript = let
     xdo = name: text: let