gitlab-ci: re-add the KDE-standard build job

Previous commits removed the shared suse-based GitLab CI build job that other KDE projects use, in favor of a custom job based on KDE Neon. This commit adds it back (while keeping the Neon one), now that the suse image has smbclient: sysadmin/ci-tooling!149 (merged)

Since dependencies are already installed in the Docker image, and the build uses all CPUs, the suse job runs ~4 times faster than the Neon one :)