CI: explicitly set the visual studio version when building for windows

.github/workflows/main.yml

@@ -53,7 +53,7 @@ jobs:
       shell: cmd
       env:
         PLATFORM: ${{ matrix.platform }}
-      run: cmake -Bbuild -Hmegasource -T v142 -A %PLATFORM% -DCMAKE_INSTALL_PREFIX=%CD%\install
+      run: cmake -Bbuild -Hmegasource -G "Visual Studio 16 2019" -A %PLATFORM% -DCMAKE_INSTALL_PREFIX=%CD%\install
     - name: Install
       shell: cmd
       run: cmake --build build --config Release --target install -j2