nixos/vscodium: fix machine usage deprecation

I decided to define the variable `machine` here, that seems cleaner than
writing `${name}.method()` everywhere.

Test fails to evaluate because of the package.

nixos/tests/vscodium.nix

@@ -51,6 +51,8 @@ let
         enableOCR = true;
 
         testScript = ''
+          machine = ${name}
+
           @polling_condition
           def codium_running():
               machine.succeed('pgrep -x codium')