ci: use local action mirrors instead of github.com

- Mirror actions/checkout, actions/cache, actions/setup-java,
  actions/upload-artifact, actions/flutter-action to local Gitea
- Replace subosito/flutter-action with actions/flutter-action
- Runner can now resolve all actions locally without internet

.github/workflows/ios-only.yml

@@ -13,7 +13,7 @@ jobs:
       - name: Install jq
         run: brew install jq
       - name: Setup Flutter
-        uses: subosito/flutter-action@v2
+        uses: actions/flutter-action@v2
         with:
           flutter-version: '3.35.7'
       - name: Get dependencies