Build on matrix of latest Ubunutu and latest Windows.

Currently that is Ubuntu 18.04 and Windows Server 2019.
https://help.github.com/en/actions/automating-your-workflow-with-github-actions/virtual-environments-for-github-hosted-runners

Author: fzs

.github/workflows/ci-build.yml

@@ -5,9 +5,10 @@ on: push
 jobs:
   build:
     name: Build and test
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
     strategy:
       matrix:
+        os: [ubuntu-latest, windows-latest]
         java-version: [8, 11]
 
     steps: