build on multiple os

Author: kerryjiang

.github/workflows/build.yml

@@ -7,7 +7,13 @@ on:
 
 jobs:
     build:
-        runs-on: ubuntu-latest
+        strategy:
+          matrix:
+            os:
+              - ubuntu-latest
+              - windows-latest
+              - macos-latest
+        runs-on: ${{matrix.os}}
         steps:
             - uses: actions/checkout@v1
             - name: Setup .NET Core