Adopt the same install-build-deps.sh the upstream Salsa-CI has started
using. Since we have our own custom build step (to support autobake-deb.sh)
we need to maintain in like this and adopt to upstream changes.

This change will make the stretch-backports build pass and use the
backported libzstd-dev 1.3.8 library (to satisfy the > 1.3.3 requirement).

Also clean away excess autopkgtest stanza and allow missing-breaks to
fail so that in total Salsa-CI would now pass.