Enable proper Linux CI

.gitlab-ci.yml

+# SPDX-FileCopyrightText: None
+# SPDX-License-Identifier: CC0-1.0
+
+include:
+  #- https://invent.kde.org/sysadmin/ci-tooling/raw/master/invent/ci-reuse.yml
+  - https://invent.kde.org/sysadmin/ci-utilities/raw/master/gitlab-templates/linux.yml