(#4) test test-third-party-build using clang

.github/workflows/ci.yml

@@ -25,8 +25,8 @@ jobs:
         run: |
           make -B
         env:
-          CC: gcc
-          CXX: g++
+          CC: clang
+          CXX: clang++
 
   build-linux-gcc:
     runs-on: ubuntu-18.04