vte.tests: fix the eval

Without the change the eval fails on `master` as:

    $ nix build --no-link -f. vte.tests
    error:
       … while evaluating the attribute 'blackbox-terminal'
         at pkgs/by-name/vt/vte/package.nix:174:7:
          173|         ;
          174|       blackbox-terminal = blackbox-terminal.override { sixelSupport = true; };
             |       ^
          175|     };

       … while calling a functor (an attribute set with a '__functor' attribute)
         at pkgs/by-name/vt/vte/package.nix:174:27:
          173|         ;
          174|       blackbox-terminal = blackbox-terminal.override { sixelSupport = true; };
             |                           ^
          175|     };

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: function 'anonymous lambda' called with unexpected argument 'sixelSupport'
       at pkgs/by-name/bl/blackbox-terminal/package.nix:1:1:
            1| {
             | ^
            2|   lib,