I believe this is caused by debootstrap - it only uses packages from the release pocket (and this is frozen from the time Ubuntu 20.04 LTS was originally released). This is a known issue https://askubuntu.com/questions/744684/latest-security-updates-with- debootstrap but I am not sure if there is much you can do to get debian- installer to say use multistrap instead of debootstrap.
** Package changed: ca-certificates (Ubuntu) => debian-installer (Ubuntu) -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1973654 Title: Using debian-installer on a server with a Let's Encrypt cert dies To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/debian-installer/+bug/1973654/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs