nixos/tests/luksroot: Fix OCR of passphrase prompt

Since a9d69a74d6edb6bcca29b1189d4bc3b203ecaf25, the passphrase prompt
now no longer starts with "Enter passphrase for" but now it's just
"Passphrase for", which causes the luksroot installer test to fail.

I've tested this on a x86_64-linux machine and the test now succeeds.

Signed-off-by: aszlig <[e-mail]>
Cc: @oxij, @samueldr
Issue: #29441

nixos/tests/installer.nix

···
       enableOCR = true;
       preBootCommands = ''
         $machine->start;
-        $machine->waitForText(qr/Enter passphrase/);
+        $machine->waitForText(qr/Passphrase for/);
         $machine->sendChars("supersecret\n");
       '';
     };