cirrus: Skip tasks on the gui repo main branch

.cirrus.yml

@@ -19,6 +19,7 @@ env:
 
 # https://cirrus-ci.org/guide/tips-and-tricks/#sharing-configuration-between-tasks
 global_task_template: &GLOBAL_TASK_TEMPLATE
+  skip: $CIRRUS_REPO_FULL_NAME == "bitcoin-core/gui" && $CIRRUS_PR == ""  # No need to run on the read-only mirror, unless it is a PR. https://cirrus-ci.org/guide/writing-tasks/#conditional-task-execution
   ccache_cache:
     folder: "/tmp/ccache_dir"
   depends_built_cache: