[test] Run CircleCI anytime (#22686)

.circleci/config.yml

@@ -206,17 +206,7 @@ workflows:
   version: 2
   pipeline:
     jobs:
-      - checkout:
-          filters:
-            branches:
-              # Ideally we only run this pipeline if the base branch is `master` or `next`.
-              # CircleCI doesn't support it because branch filters are based on target branch.
-              # We approximate it by running on `master`, `next` and any PR (assuming they always target master or next).
-              only:
-                - master
-                - next
-                # pull requests have its branch name following the `pull/$CIRCLE_PR_NUMBER` scheme.
-                - /pull\/.*/
+      - checkout
       - test_unit:
           requires:
             - checkout