Update ci.yml

Author: jlduan

.github/workflows/ci.yml

@@ -5,12 +5,12 @@ name: ci
 on:
   push:
     branches:
-      - $default-branch
+      - master
     paths-ignore:
       - 'docs/**'
   pull_request:
     branches:
-      - $default-branch
+      - master
   schedule:
     # * is a special character in YAML so you have to quote this string
     - cron:  '3 14 15 * *'
@@ -26,7 +26,7 @@ jobs:
     steps:
     - uses: actions/checkout@v2
       with:
-        ref: ''
+        ref: 'master'
     - name: Set up Python 3.8
       uses: actions/setup-python@v2
       with: