Add toSecretFile primitive for writing a file which is only readable by the owner of the nix-store.
Move userName responsibility to callers of writeDerivation.
Check the secretDerivation property to make the derivation secret.
Ensure that newer versions of Nix can communicate with older versions of the nix-daemon.
Enforce definition of a SecretMode around canonicalise functions.
Mark the derivation outputs as secret if the derivation file is secret.