v4.4.0 — drop .gitlab-ci.yml from build_change_paths + optional needs on promote

Removes .gitlab-ci.yml from default build_change_paths in
base-build-scratch.yml and instance.yml so non-image-affecting pushes
to main no longer trigger the bootc cascade.

Bundled fix for the sibling empty-pipeline failure: promote jobs now
have optional: true on their needs of the build job, plus a runtime
guard that exits 0 silently when the SHA-tagged image doesn't exist.

See CHANGELOG.md for details.