test if it builds on all platforms

Author: loanstone

.github/workflows/dotnet.yml

@@ -12,7 +12,11 @@ on:
 jobs:
   build:
 
-    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
+
+    runs-on: ${{ matrix.os }}
 
     steps:
     - uses: actions/checkout@v4