[CI] name of github default branch

.gitlab-ci.yml

@@ -30,7 +30,7 @@ update_repository:
     - git fetch -p origin
     - git remote add mantid https://github.com/mantidproject/mantid.git
     - git fetch -p mantid
-    - git pull mantid master
+    - git pull mantid main
     - git push origin HEAD:ill-main
   allow_failure: true