gitlab-ci: Do not trigger crossbuilds anymore

febb2126 was proposed temporarely while we had hope of having a fix.
Now, we have !1860 (apparently 99% done) but the main maintainer of
crossbuilds refused to look at it because he is focused on core code
until 3.0 so, with this timeframe, the commit lost its reason to exist.

So, to avoid keeping runners work unnecessarelly with abandoned jobs,
let's not trigger them ('when: never') as I proposed on IRC months ago.

.gitlab-ci.yml

@@ -375,8 +375,11 @@ gimp-flatpak-x64:
   extends: .default
   rules:
     - <<: *CI_MERGE
+      when: never
     - <<: *CI_COMMIT
+      when: never
     - if: '$GIMP_CI_CROSSROAD_WIN64 != null'
+      when: never
   image: $CI_REGISTRY_IMAGE:build-debian-latest
 
 deps-win-x64-cross:
@@ -409,7 +412,6 @@ gimp-win-x64-cross:
   stage: gimp
   script:
     - bash build/windows/2_build-gimp-crossroad.sh
-  allow_failure: true
   artifacts:
     expose_as: 'Windows zip'
     paths: