Update linux-builds.yml

Author: githubuser0xFFFF

.github/workflows/linux-builds.yml

@@ -4,8 +4,11 @@ on: [push]
 
 jobs:
   build:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, ubuntu-18.04, ubuntu-16.04]
 
-    runs-on: ubuntu-18.04
+    runs-on: ${{ matrix.os }}
 
     steps:
     - uses: actions/checkout@v1