CI: allow image rebuilding job only in pipelines belonging to default branch

.gitlab-ci.yml

@@ -50,7 +50,11 @@ update-build-env:  # This job updates building environment image. Run it if FPC
                    --build-arg FPC_MAIN_VER=$FPC_MAIN_VER
                    -t $IMAGE_TAG - < Dockerfile.buildenv
     - docker push $IMAGE_TAG
-  when: manual
+  rules:
+    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
+      when: manual
+      allow_failure: true
+    - when: never
 
 update-qt5pas-bindings:  # build and cache Qt5 Pascal bindings
   stage: prepcaches