gitlab-ci: fix (again!) the gitlab-ci.

Argh! So it turns out that .publish_nightly template uses already an
only: key so we cannot use rules: (on the other hand I guess using only:
ourselves is alright and concatenate ours and the one in the template).

Fixes:
> jobs:flatpak-nightly config key may not be used with `rules`: only

.gitlab-ci.yml

@@ -810,9 +810,12 @@ win32-nightly:
 flatpak-nightly:
   extends: '.publish_nightly'
   stage: distribution
-  rules:
-    # Custom builds though web GUI, API or schedules.
-    - if: '$GIMP_CI_FLATPAK != null'
+  only:
+    variables:
+      # Custom builds though web GUI, API or schedules.
+      # I don't use 'rules:' because .publish_nightly template uses an
+      # 'only:' already which clashes with 'rules:'.
+      - $GIMP_CI_FLATPAK != null
   needs: ["flatpak"]
   dependencies:
     - 'flatpak'