ci: Run vcpkg with path prefix

The GHA VS installation includes its own vcpkg package manager, which is
available since VS 17.6. This change avoids any ambiguity about which
copy of vcpkg we run.

.github/workflows/ci.yml

@@ -279,7 +279,7 @@ jobs:
         run: |
           Set-Location "$env:VCPKG_INSTALLATION_ROOT"
           Add-Content -Path "triplets\x64-windows-static.cmake" -Value "set(VCPKG_BUILD_TYPE release)"
-          vcpkg --vcpkg-root "$env:VCPKG_INSTALLATION_ROOT" integrate install
+          .\vcpkg.exe --vcpkg-root "$env:VCPKG_INSTALLATION_ROOT" integrate install
           git rev-parse HEAD | Out-File -FilePath "$env:GITHUB_WORKSPACE\vcpkg_commit"
           Get-Content -Path "$env:GITHUB_WORKSPACE\vcpkg_commit"