Add Linux Qt6 CI

The Plasma 6 seed job is now in place and working, so CI for Plasma repos
depending on other Plasma modules is now also possible.

.gitlab-ci.yml

@@ -4,3 +4,4 @@
 include:
   - https://invent.kde.org/sysadmin/ci-utilities/raw/master/gitlab-templates/linux.yml
   - https://invent.kde.org/sysadmin/ci-utilities/raw/master/gitlab-templates/freebsd.yml
+  - https://invent.kde.org/sysadmin/ci-utilities/raw/master/gitlab-templates/linux-qt6.yml