nixos/tests/luksroot: Fix OCR of passphrase prompt

Since a9d69a74

, 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 <aszlig@nix.build>
Cc: @oxij, @samueldr
Issue: #29441