Actually use the OS in matrix

Author: headius

.github/workflows/ruby.yml

@@ -14,7 +14,7 @@ jobs:
         os: [ ubuntu-latest, windows-latest, macos-latest ]
         ruby: [ jruby-9.4.12.0, jruby-head ]
 
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
 
     steps:
     - uses: actions/checkout@v2