Set compiler for OSRM build

Author: kkarbowiak

.github/workflows/vroom_libosrm.yml

@@ -51,7 +51,7 @@ jobs:
         run: |
           mkdir build
           cd build
-          cmake .. -DCMAKE_BUILD_TYPE=Release
+          cmake .. -DCMAKE_BUILD_TYPE=Release -DCMAKE_CXX_COMPILER=${{ matrix.cxx }}
           cmake --build .
         working-directory: osrm-backend
       - name: Install OSRM