Revert "ci: Temporarily allow build-sysext CI job to fail"

This reverts commit aacc968f0e.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3570>

.gitlab-ci.yml

@@ -12,11 +12,6 @@ include:
         -Dextensions-tool:bash_completion=disabled
 
 build-sysext:
-  # FIXME: Temporarily allow this to fail, as it is basically impossible for it
-  # to run successfully while GitLab is broken, due to the number of sequential
-  # git clone operations it does.
-  # See https://discourse.gnome.org/t/scheduled-maintenance-gitlab-gitlab-pages-04th-of-december-2024-at-2-pm-utc/25271
-  allow_failure: true
   tags:
     - x86_64_v3
   before_script: