lego.tests: fix the eval

Without the change the eval fails as:

    $ nix build --no-link -f. lego.tests
    error:
       … while evaluating the attribute 'lego-dns'
         at pkgs/by-name/le/lego/package.nix:34:5:
           33|     lego-http = nixosTests.acme.http01-builtin;
           34|     lego-dns = nixosTests.acme.dns01-builtin;
             |     ^
           35|   };

       … while evaluating the attribute 'acme.dns01-builtin'
         at nixos/tests/all-tests.nix:149:3:
          148|   aaaaxy = runTest ./aaaaxy.nix;
          149|   acme = import ./acme/default.nix { inherit runTest; };
             |   ^
          150|   acme-dns = handleTest ./acme-dns.nix {};

       error: attribute 'dns01-builtin' missing
       at pkgs/by-name/le/lego/package.nix:34:16:
           33|     lego-http = nixosTests.acme.http01-builtin;
           34|     lego-dns = nixosTests.acme.dns01-builtin;
             |                ^
           35|   };