ci: limit branches to master to avoid duplicated runs

Signed-off-by: Shengqi Chen <harry-chen@outlook.com>

.github/workflows/tunasync.yml

@@ -1,6 +1,11 @@
 name: tunasync
 
-on: [push, pull_request, workflow_dispatch]
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+  workflow_dispatch:
 
 jobs: