live-build uses wrong security suite name for Bullseye
live-build will output the old security suite name (bullseye/updates), not the new one introduced in Bullseye (bullseye-security). This is actually supposed to work on Debian's repo's, so if we can configure our Debian mirror repos to accept it that's a fix.
Otherwise we need a tiny fix in live-build. It could be as simple as the current oneline fix that is applied on-the-fly (3e3da52e). But we probably want a slightly more elaborate fix that outputs one or the other depending on if we are building bullseye or earlier, so the same live-build package can be used to build all our branches.