fix compiler env

Author: andreasbuhr

.github/workflows/cmake.yml

@@ -32,7 +32,7 @@ jobs:
       shell: bash
       working-directory: ${{runner.workspace}}/build
       env: 
-        CC: ${{ matrix.compiler }}
+        CXX: ${{ matrix.compiler }}
       # Note the current convention is to use the -S and -B options here to specify source 
       # and build directories, but this is only available with CMake 3.13 and higher.  
       # The CMake binaries on the Github Actions machines are (as of this writing) 3.12