build system: added older compiler versions to the CI

.github/workflows/build.yml

@@ -9,7 +9,10 @@ jobs:
 
     strategy:
       matrix:
-        compiler: [g++, clang++]
+        compiler: [
+            g++-8, g++-9, g++,
+            clang++-8, clang++-9, clang++
+        ]
 
     runs-on: ubuntu-latest