gitlab-ci: try to fix the CI for releases (and other CI pipelines).

.gitlab-ci.yml

@@ -226,12 +226,14 @@ gimp-distcheck-debian:
 
 gimp-meson-debian:
   rules:
-    # On commits except tags.
-    - if: '$CI_PIPELINE_SOURCE == "push" && $CI_COMMIT_TAG == null'
-    # On merge requests.
+    # On commits and merge requests.
+    - if: '$CI_PIPELINE_SOURCE == "push"'
     - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
+    # On releases.
+    - if: '$CI_COMMIT_TAG != null'
     # Custom builds though web GUI, API or schedules.
     - if: '$GIMP_CI_MESON_GCC != null'
+    - if: '$GIMP_CI_SOURCES != null'
   extends: .gimp-debian-base
   script:
     - meson _build