Run test job at 06:00 every day

Well, it seems the environment is changing so we'd better run the tests
reguarly to see whether something breaks in a timely manner.

GitHub Actions also don't offer other Windows environments now, only
windows-latest is available. Reduce test matrix to only that.

.github/workflows/main.yml

@@ -6,15 +6,15 @@ on:
     branches:
       - master
       - release/*
+  schedule:
+    - cron: '0 6 * * *'
 
 jobs:
   test:
     name: default
-    runs-on: ${{ matrix.os }}
+    runs-on: windows-latest
     strategy:
       fail-fast: false
-      matrix:
-        os: [windows-2016, windows-2019, windows-latest]
     steps:
       - name: Check out source code
         uses: actions/checkout@v1