gitlab-ci: fix the CI.

The flatpak_ci_initiative.yml template we include got changed 2 days ago,
replacing the "only" by a "rules", which broke our CI file.

Cf. commit 193f63c20f87d38868c05beee6446b387b73e140 in repository citemplates.

The present commit should (hopefully) fix this error:

> Unable to create pipeline
>   jobs:flatpak-nightly config key may not be used with `rules`: only

(even though the error is a bit cryptic, the problem is apparently that both
rules: and only: keys cannot be used together)

.gitlab-ci.yml

@@ -1015,12 +1015,9 @@ win32-nightly:
 flatpak-nightly:
   extends: '.publish_nightly'
   stage: distribution
-  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:'.
-      - $GIMP_CI_FLATPAK != null
+  rules:
+    # Custom builds though web GUI, API or schedules.
+    - if: '$GIMP_CI_FLATPAK != null'
   needs: ["flatpak"]
   dependencies:
     - 'flatpak'