.gitlab-ci.yml: Use buildstream 1.4.3
core-deps/sane-backends is returning 403's in the CI track, see:
https://gitlab.gnome.org/GNOME/gnome-build-meta/-/jobs/718983
Curling https://gitlab.com/sane-project/backends/uploads/a3ba9fff29253a94e84074917bff581a/sane-backends-1.0.27.tar.gz
locally, or tracking locally with bst 1.4.3 works fine.
(ID taken from https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/commit/7b6d1ec757f3e7bec8051d07f073ced4daf6f432, with release including useragent change https://gitlab.com/BuildStream/buildstream/-/merge_requests/1898)
Edited by Tom Pollard