Correcting my NixOS LUKS mistakes

I have previously written about how I setup full disk encryption via LUKS on my NixOS machines. There I made the mistake of not initializing a filesystem on the (encrypted) partition meant to hold the key to unlock the other disks. This apparently only worked on NixOS on the happenstance that systemd was patched up to make it work. Recently (just before the NixOS 26.05 release) that patch was…