ci: Run nightly job even if cross environment stage fails

Currently, the nightly job is skipped if fedora rawhide, or flatpak
master job fails. This is not wanted as the nightly should be published
if flatpak devel job succeeds. Let's allow failures of those jobs to
not block the deploy stage.

.gitlab-ci.yml

@@ -36,6 +36,7 @@ fedora rawhide:
     only:
         - web
         - master
+    allow_failure: true
 
 flatpak master:
     stage: cross environment
@@ -51,6 +52,7 @@ flatpak master:
         - schedules
         - web
         - tags
+    allow_failure: true
 
 triage:
     image: ruby:2.4