nixos/sks: Fix the module (the pre-start script was broken)

Unfortunately the changes in ab5dcc70
introduced a typo (took me a while to spot that...) that broke the
whole module (or at least the sks-db systemd unit).

The systemd unit was failing with the following error message:
...-unit-script-sks-db-pre-start[xxx]: KDB/DB_CONFIG exists but is not a symlink.