specify opencv dir

Author: sdcb

.github/workflows/opencvsharp.yml

@@ -47,7 +47,7 @@ jobs:
         cd opencvsharp && mkdir build && cd build && \
         cmake \
           -D CMAKE_INSTALL_PREFIX=${{ github.workspace }}/opencvsharp/build \
-          -D OpenCV_DIR=${{ github.workspace }}/opencv \
+          -D OpenCV_DIR=${{ github.workspace }}/opencv/lib/cmake/opencv4 \
           ../src
 
     - name: Build OpenCvSharp