nixos/tests/installer: Fix matching LUKS prompt

The LUKS passphrase prompt has changed from "Enter passphrase" to "Enter
LUKS Passphrase" in c69c76ca

, so the OCR
detection of the test fails indefinitely.

Unfortunately, this doesn't fix the test because we have a real problem
here:

Enter LUKS Passphrase:
killall: cryptsetup: no process killed
Enter LUKS Passphrase:

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @abbradar