zlib needs to be found (with vcpkg in CI), do not use our bundled copy

.github/workflows/unittest_windows.yml

@@ -10,8 +10,10 @@ jobs:
     steps:
     - uses: actions/checkout@v1
     - uses: seanmiddleditch/gha-setup-vsdevenv@master
+    - run: |
+        vcpkg install zlib:x64-windows
     - run: |
         mkdir build
         cd build
-        cmake -DCMAKE_CXX_COMPILER=cl.exe -DUSE_WS=1 -DUSE_TEST=1 ..
+        cmake -DCMAKE_TOOLCHAIN_FILE=c:/vcpkg/scripts/buildsystems/vcpkg.cmake -DCMAKE_CXX_COMPILER=cl.exe -DUSE_WS=1 -DUSE_TEST=1 ..
     - run: cmake --build build