Update build.net-core.yml

.github/workflows/build.net-core.yml

@@ -1,11 +1,12 @@
 name: .NET Core CI
-
-on: [push]
-
+on: [push, pull_request]
 jobs:
   build:
     name: Build
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest, macOS-latest]
     steps:
     - uses: actions/checkout@master
     - name: Setup .NET Core