nixos/tests/sddm: Fix detecting login screen

Tesseract seems to have a hard time detecting the "ALICE FOOBAR" text,
so let's match on "Select your user and enter password" instead.

Ran the test on x86_64-linux and it now succeeds.

Signed-off-by: aszlig <[e-mail]>

nixos/tests/sddm.nix

···
         user = nodes.machine.config.users.extraUsers.alice;
       in ''
         startAll;
-        $machine->waitForText(qr/ALICE/);
+        $machine->waitForText(qr/select your user/i);
         $machine->screenshot("sddm");
         $machine->sendChars("${user.password}\n");
         $machine->waitForFile("/home/alice/.Xauthority");