Update dotnet.yml

added macos-latest and windows-latest to runs-on

Author: seeste

.github/workflows/dotnet.yml

@@ -8,9 +8,10 @@ on:
 
 jobs:
   build:
-
-    runs-on: ubuntu-latest
-
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [macos-latest, ubuntu-latest, windows-latest]
     steps:
     - uses: actions/checkout@v2
     - name: Setup .NET