ubuntu20: install zfs from bullseye-backports (on recovery)

Hetzner's current recovery uses debian, Ubuntu PPA
packages don't work. This simplifies things.

hetzner-ubuntu20-zfs-setup.sh

@@ -461,13 +461,8 @@ done
 
 echo "======= installing zfs on rescue system =========="
  echo "zfs-dkms zfs-dkms/note-incompatible-licenses note true" | debconf-set-selections
- apt-get install --yes software-properties-common
- apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 8CF63AD3F06FC659
- add-apt-repository 'deb http://ppa.launchpad.net/jonathonf/zfs/ubuntu focal main'
- apt update
- apt install --yes zfs-dkms zfsutils-linux
- add-apt-repository -r 'deb http://ppa.launchpad.net/jonathonf/zfs/ubuntu focal main'
  apt update
+ apt install -t bullseye-backports --yes zfs-dkms zfsutils-linux
  find /usr/local/sbin/ -type l -exec rm {} +
  zfs --version