enable auto sync fork

.github/workflows/sync-fork.yml

@@ -0,0 +1,30 @@
+name: Sync Fork
+
+on:
+  schedule:
+    - cron: '0 2 * * *'
+  workflow_dispatch:
+
+jobs:
+  sync:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+        with:
+          ref: 'master'
+          fetch-depth: 0
+
+      - name: Configure Git
+        run: |
+          git config --global user.name 'GitHub Actions'
+          git config --global user.email 'actions@github.com'
+
+      - name: Merge upstream changes
+        run: |
+          git remote add upstream https://github.com/sbwml/r4s_build_script.git
+          git fetch upstream
+          git merge upstream/master --allow-unrelated-histories --no-edit || exit 1
+
+      - name: Push changes
+        run: git push