Add the two available OS (#306)

.github/workflows/cmake.yml

@@ -5,6 +5,7 @@ on:
     branches: [ master ]
   pull_request:
     branches: [ master ]
+  workflow_dispatch:
 
 env:
   # Customize the CMake build type here (Release, Debug, RelWithDebInfo, etc.)
@@ -16,7 +17,11 @@ jobs:
     # well on Windows or Mac.  You can convert this to a matrix build if you need
     # cross-platform coverage.
     # See: https://docs.github.com/en/free-pro-team@latest/actions/learn-github-actions/managing-complex-workflows#using-a-build-matrix
-    runs-on: ubuntu-latest
+    runs-on: ${{matrix.os}}
+
+    strategy:
+      matrix:
+        os: [ubuntu-18.04, ubuntu-20.04]
 
     steps:
     - uses: actions/checkout@v2