ci: specify .sln argument correctly

The path has spaces...

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

.github/workflows/msbuild.yml

@@ -21,7 +21,7 @@ jobs:
 
     - name: Build
       working-directory: ${{env.GITHUB_WORKSPACE}}
-      run: msbuild /m /p:Configuration=${{env.BUILD_CONFIGURATION}} ${{env.SOLUTION_FILE_PATH}}
+      run: msbuild /m /p:Configuration=${{env.BUILD_CONFIGURATION}} "${{env.SOLUTION_FILE_PATH}}"
 
     - name: Upload `wintoast.exe`
       uses: actions/upload-artifact@v3