Upstream Salsa-CI refactored the build process in
https://salsa.debian.org/salsa-ci-team/pipeline/-/commit/58880fcef5b742cb9c661121a8c8707bf392b3b5

This broke our custom direct invocation of install-build-deps.sh as the
Salsa-CI images no longer contain them. Adapt the .build-script
equivalent to follow new Salsa-CI method so builds work again.