DEBIAN_STABLE: use buster-backports instead of stretch-backports

Debian/buster is the current stable release, there's no point in
enabling stretch-backports there.

FTR: we could set it to 'stable-backports' instead, though then
apt/apt-get/... will complain about it:

| W: Conflicting distribution: http://ftp.debian.org/debian stable-backports InRelease (expected stable-backports but got buster-backports)