Build/CI: further refine clang build workflow

.github/workflows/clang-build.yml

@@ -11,8 +11,9 @@ jobs:
     - uses: actions/checkout@v2
     - name: Dependencies
       run: |
-        sudo apt-get update && sudo apt-get install -yq libboost-all-dev clang-11
-        sudo update-alternatives --install /usr/bin/cc cc /usr/bin/clang-11 100 --slave /usr/bin/c++ c++ /usr/bin/clang++-11
+        sudo apt-get update && sudo apt-get install -yq libboost-all-dev clang gcc g++
+        sudo update-alternatives --install /usr/bin/cc cc /usr/bin/clang 100
+        sudo update-alternatives --install /usr/bin/c++ c++ /usr/bin/clang 100
     - name: Setup
       run: |
         mkdir bin