Comment 8 for bug 1511938

@mapreri, we have some code to deal with this already in pbuilder:

pbuilder-apt-config:70
      dapper|hardy|intrepid|jaunty|karmic|lucid)
        case "$arch" in
          amd64|i386)
            echo "ubuntu"
          ;;
          armel|armhf|hppa|ia64|lpia|powerpc|sparc)
            echo "ubuntu-ports"
          ;;
          *)
            die "Unknown Ubuntu archive for arch=$arch"
          ;;
        esac

so, why don't use pbuilder-apt-config to get the correct sources.lists file?