ci: Switch to VS-vendored vcpkg instance

The GHA Windows image contains two vcpkg installations: one vendored by
VS (`VCPKG_ROOT`) and an independent one (`VCPKG_INSTALLATION_ROOT`).
Our build documentation recommends using the VS-vendored instance to
minimize user effort. While this option previously failed to build in
the CI, forcing us to use the independent installation, it is now
working correctly.

This change removes the no-longer-needed workaround.

.github/workflows/ci.yml

@@ -253,15 +253,11 @@ jobs:
           py -3 --version
           bash --version
 
-      - name: Set VCPKG_ROOT
-        run: |
-          echo "VCPKG_ROOT=${VCPKG_INSTALLATION_ROOT}" >> "$GITHUB_ENV"
-
       - name: Restore vcpkg tools cache
         id: vcpkg-tools-cache
         uses: actions/cache/restore@v5
         with:
-          path: C:/vcpkg/downloads/tools
+          path: ~/AppData/Local/vcpkg/downloads/tools
           key: ${{ github.job }}-vcpkg-tools-${{ github.run_id }}
           restore-keys: ${{ github.job }}-vcpkg-tools-
 
@@ -288,7 +284,7 @@ jobs:
         # Only save cache from one job as they share tools. If the matrix is expanded to jobs with unique tools, this may need amending.
         if: github.event_name != 'pull_request' && github.ref_name == github.event.repository.default_branch && steps.vcpkg-tools-cache.outputs.cache-hit != 'true' && matrix.job-type == 'standard'
         with:
-          path: C:/vcpkg/downloads/tools
+          path: ~/AppData/Local/vcpkg/downloads/tools
           key: ${{ github.job }}-vcpkg-tools-${{ github.run_id }}
 
       - name: Build