ci: update sync fork workflow (#424)

.github/workflows/sync_fork.yml

@@ -2,6 +2,9 @@ name: Sync fork
 
 on:
   workflow_dispatch:
+  workflow_run:
+    workflows: [Sync beta branch]
+    types: [completed]
   pull_request:
     branches: [develop]
     types: [opened, synchronize, reopened]
@@ -16,7 +19,7 @@ jobs:
       - uses: actions/checkout@v3
         with:
           fetch-depth: 0
-          ref: ${{ github.head_ref || github.ref_name }}
+          ref: ${{ github.event_name == 'workflow_run' && 'beta' || github.head_ref || github.ref_name }}
       - uses: webfactory/[email protected]
         with:
           ssh-private-key: ${{ secrets.FORK_KEY }}