[ci] Fox doctest integration on Windows

Author: serge-sans-paille

.github/workflows/windows.yml

@@ -81,14 +81,13 @@ jobs:
             cc:p
             cmake:p
             ninja:p
-            doctest:p
       - name: Checkout xsimd
         uses: actions/checkout@v2
       - name: Configure
         run: |
           mkdir _build
           cd _build
-          cmake .. -DBUILD_TESTS=ON -DBUILD_BENCHMARK=ON -DBUILD_EXAMPLES=ON -DCMAKE_BUILD_TYPE=Release -G Ninja
+          cmake .. -DBUILD_TESTS=ON -DBUILD_BENCHMARK=ON -DBUILD_EXAMPLES=ON -DCMAKE_BUILD_TYPE=Release -DDOWNLOAD_DOCTEST=ON -G Ninja
       - name: Build
         run: ninja -C _build
       - name: Test