Connect vcpkg to cmake using a toolchain file

There is a mechanism to supply a "real" toolchain even when vcpkg is
supplying one. Google details if required.

.github/workflows/ci.yml

@@ -66,6 +66,7 @@ jobs:
       # See https://cmake.org/cmake/help/latest/variable/CMAKE_BUILD_TYPE.html?highlight=cmake_build_type
       run: >
         cmake -B ${{ steps.strings.outputs.build-output-dir }}
+        -DCMAKE_TOOLCHAIN_FILE=${{ github.workspace }}/scripts/buildsystems/vcpkg.cmake
         -DCMAKE_CXX_COMPILER=${{ matrix.cpp_compiler }}
         -DCMAKE_C_COMPILER=${{ matrix.c_compiler }}
         -DCMAKE_BUILD_TYPE=${{ matrix.build_type }}