CI: Explicitly disable GTEST_DOWNLOAD to fix client-only builds

Author: Kaffeine

.github/workflows/build.yml

@@ -106,7 +106,7 @@ jobs:
       run: |
         mkdir release
         cd release
-        ${{ matrix.cmake-path }}cmake -E env ${{ matrix.cmake-init-env }} ${{ matrix.cmake-path }}cmake ${{ matrix.cmake-args }} -DCMAKE_BUILD_TYPE=Release -DTOOLS=OFF -DSERVER=OFF  -DAUTOUPDATE=ON -Werror=dev -DCMAKE_RUNTIME_OUTPUT_DIRECTORY_RELEASE=. ..
+        ${{ matrix.cmake-path }}cmake -E env ${{ matrix.cmake-init-env }} ${{ matrix.cmake-path }}cmake ${{ matrix.cmake-args }} -DCMAKE_BUILD_TYPE=Release -DTOOLS=OFF -DSERVER=OFF -DDOWNLOAD_GTEST=OFF -DAUTOUPDATE=ON -Werror=dev -DCMAKE_RUNTIME_OUTPUT_DIRECTORY_RELEASE=. ..
         ${{ matrix.cmake-path }}cmake --build . --config Release --target everything ${{ matrix.build-args }}
 
     - name: Package