Test on Windows and macOS because for me 'mvn' passes on Windows but

GitHub fails on Ubuntu.

.github/workflows/maven.yml

@@ -20,11 +20,12 @@ on: [push, pull_request]
 jobs:
   build:
 
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
     continue-on-error: ${{ matrix.experimental }}
     strategy:
       fail-fast: false
       matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
         java: [ 8, 11, 17 ]
         experimental: [false]
 #        include: