ci: Rely on cmake --preset toolchain file

This is the standard approach and avoids relying on
VCPKG_INSTALLATION_ROOT and -DCMAKE_TOOLCHAIN_FILE= in the ci-windows.py
script.

This makes it easier to run locally.

.github/workflows/ci.yml

@@ -255,6 +255,10 @@ jobs:
           # Workaround for libevent, which requires CMake 3.1 but is incompatible with CMake >= 4.0.
           sed -i '1s/^/set(ENV{CMAKE_POLICY_VERSION_MINIMUM} 3.5)\n/' "${VCPKG_INSTALLATION_ROOT}/scripts/ports.cmake"
 
+      - name: Set VCPKG_ROOT
+        run: |
+          echo "VCPKG_ROOT=${VCPKG_INSTALLATION_ROOT}" >> "$GITHUB_ENV"
+
       - name: vcpkg tools cache
         uses: actions/cache@v5
         with: