gistfile1.txt

  system.activationScripts.postActivation.text = ''
    printf "disabling spotlight indexing... "
    mdutil -i off -d / &> /dev/null
    mdutil -E / &> /dev/null
    echo "ok"

    printf "configuring ssh keys for hydra on the root account... "
    mkdir -p ~root/.ssh
    cp -f /etc/per-user/root/ssh/authorized_keys ~root/.ssh/authorized_keys
    chown root:wheel ~root ~root/.ssh ~root/.ssh/authorized_keys
    echo "ok"

    if [ ! -L /run ]; then
        sudo ln -s /private/var/run /run
    fi

    printf "moving the oldbashrc and nixconf out of the way ..."
    if [ ! -f /etc/bashrc.old ]; then
      mv /etc/bashrc /etc/bashrc.old
    fi
    if [ ! -f /etc/nix/nix.conf.old ]; then
      mv /etc/nix/nix.conf /etc/nix/nix.conf.old
    fi
    echo "ok"

  '';

This will override options like nix.nixPath because it runs later,
remove this snippet from /etc/profile:

    # Nix
    if [ -e '/nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh' ]; then
      . '/nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh'
    fi
    # End Nix