Fix --with-libarchive path: hardcode C:\vcpkg installed dir

env context in GitHub Actions only covers variables set in the
workflow itself, not runner system env vars, so
\${{ env.VCPKG_INSTALLATION_ROOT }} expanded to empty string.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: cataphract

.github/workflows/tests.yml

@@ -107,7 +107,7 @@ jobs:
           php-version: ${{ matrix.php }}
           arch: ${{ matrix.arch }}
           ts: ${{ matrix.ts }}
-          args: --with-libarchive=${{ env.VCPKG_INSTALLATION_ROOT }}\installed\x64-windows-static-md
+          args: --with-libarchive=C:\vcpkg\installed\x64-windows-static-md
 
       - name: Upload test results
         uses: actions/upload-artifact@v4