Change push CI to run on workflow_run event

.github/workflows/self-push-caller.yml

@@ -1,3 +1,4 @@
+# Used to trigger self-push CI
 name: Self-hosted runner (push-caller)
 
 on:
@@ -17,13 +18,4 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout transformers
-        uses: actions/checkout@v2
-        with:
-          fetch-depth: 2
-          ssh-key: "${{ secrets.COMMIT_KEY }}"
-
-      - name: Checkout to branch push-ci
-        # A more strict way to make sure`push-ci` is exactly the same as `main` at the push event commit.
-        run: |
-          git checkout -b push-ci
-          git push -u origin push-ci --force
+        run: echo "Trigger push CI via workflow_run"

.github/workflows/self-push.yml

@@ -1,9 +1,12 @@
 name: Self-hosted runner (push)
 
 on:
+  workflow_run:
+    workflows: ["Self-hosted runner (push-caller)"]
+    branches: ["main"]
+    types: [completed]
   push:
     branches:
-      - push-ci
       - ci_*
       - ci-*
     paths: