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/ci-windows.py

@@ -39,12 +39,6 @@ GENERATE_OPTIONS = {
 
 
 def generate(ci_type):
-    toolchain_file = os.path.join(
-        os.environ["VCPKG_INSTALLATION_ROOT"],
-        "scripts",
-        "buildsystems",
-        "vcpkg.cmake",
-    )
     command = [
         "cmake",
         "-B",
@@ -52,7 +46,6 @@ def generate(ci_type):
         "-Werror=dev",
         "--preset",
         "vs2022",
-        f"-DCMAKE_TOOLCHAIN_FILE={toolchain_file}",
     ] + GENERATE_OPTIONS[ci_type]
     run(command)
 

.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: