Otherwise the build fails after debootstrap has done its job
and live-build tries to use apt-key:

  I: Base system installed successfully.
  P: Begin caching bootstrap stage...
  P: Begin unmounting filesystems...
  P: Setting up cleanup function
  P: Begin caching chroot stage...
  P: Begin mounting /dev/pts...
  P: Begin mounting /proc...
  P: Begin mounting /sys...
  P: Configuring file /etc/debian_chroot
  P: Configuring file /sbin/start-stop-daemon
  P: Configuring file /usr/sbin/policy-rc.d
  P: Configuring file /usr/sbin/initctl
  P: Configuring file /etc/hosts
  P: Configuring file /etc/resolv.conf
  P: Configuring file /etc/hostname
  P: Configuring file /bin/hostname
  P: Configuring file /etc/apt/apt.conf
  P: Configuring file /etc/apt/sources.list
  E: gnupg, gnupg2 and gnupg1 do not seem to be installed, but one of them is required for this operation