tweak github-actions.yml

.github/workflows/github-actions.yml

@@ -1,12 +1,18 @@
 name: CI
-on: [push]
+on:
+  push:
+    branches:
+      - main
 
 jobs:
   build:
     name: Build
     runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+          ref: master
       - uses: actions/setup-python@v5
         with:
           python-version: '3.12'