Update build.yml

Author: paulofaria

.github/workflows/build.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [macos-latest]
+        os: [macos-10.15, ubuntu-16.04, ubuntu-18.04, ubuntu-20.04]
     steps:
     - uses: actions/checkout@v2
     - name: Build