fix cc and cxx variables

.github/workflows/cmake.yml

@@ -61,7 +61,7 @@ jobs:
       # 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
-      run: cmake $GITHUB_WORKSPACE -DCMAKE_BUILD_TYPE=$BUILD_TYPE -DCMAKE_C_COMPILER=${{ matrix.config.cc }} -DCMAKE_CXX_COMPILER=${{ matrix.config.cxx }}
+      run: cmake $GITHUB_WORKSPACE -DCMAKE_BUILD_TYPE=$BUILD_TYPE -DCMAKE_C_COMPILER=${{ matrix.cc }} -DCMAKE_CXX_COMPILER=${{ matrix.cxx }}
 
     - name: Build
       working-directory: ${{github.workspace}}/build