Proves the sops-nix pipeline end to end on mibook: encrypts a
throwaway secret to the operator + mibook host keys, declares it in
machines/mibook/secrets.nix, and verifies it decrypts to
/run/secrets/placeholder (root:root, 0400) on nixos-rebuild switch.
Also removes the dead `let secrets = config.sops.secrets;` binding
from both environments.nix files — leftover from an earlier abandoned
plan, never referenced.
finn.markwitz
a00916864c