CI: Fix rules to keep running pipelines on master
Not doing so will eventually lead to issues with prebuilt contribs when the pipeline doesn't run in the videolan/vlc namespace
This is a regression introduced in !1477 (merged) because of an invalid pattern for the target branch.
FWIW, you can see the environment variables...
- available in the MR here: https://code.videolan.org/-/snippets/1411
- once the MR has been merged and the pipeline runs on master here: https://code.videolan.org/-/snippets/1412