CI - workaround 'doc' task error caused by new choco 'graphviz' package.

.github/workflows/main.yml

@@ -238,7 +238,7 @@ jobs:
         uses: actions/checkout@v2
         with: { fetch-depth: 0 }
       - name: Install dependencies
-        run: do { choco install --no-progress doxygen.install graphviz } until ($?)
+        run: do { choco install --no-progress doxygen.install graphviz.portable } until ($?)
       - name: CMake
         run: rake cmake
       - name: Build