ci: Fix traceback when user doesn't have a cerbero fork

```
Traceback (most recent call last):
  File "ci/gitlab/trigger_cerbero_pipeline.py", line 59, in <module>
    if cerbero is None:
NameError: name 'cerbero' is not defined
```

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3063>

diff --git a/ci/gitlab/trigger_cerbero_pipeline.py b/ci/gitlab/trigger_cerbero_pipeline.py
index 7cccb23..303fa09 100755 (executable)
--- a/ci/gitlab/trigger_cerbero_pipeline.py
+++ b/ci/gitlab/trigger_cerbero_pipeline.py
@@ -47,6 +47,7 @@ if __name__ == "__main__":
                 return cerbero
         return None
 
+    cerbero = None
     # We do not want to run on (often out of date) user upstream branch
     if os.environ["CI_COMMIT_REF_NAME"] != os.environ['GST_UPSTREAM_BRANCH']:
         try: