Try this.

.github/workflows/windows.yml

@@ -58,9 +58,9 @@ jobs:
       - name: Install Basix (C++)
         working-directory: basix/cpp
         run: |
-          cmake -DINSTALL_RUNTIME_DEPENDENCIES=ON -DCMAKE_TOOLCHAIN_FILE=C:/vcpkg/scripts/buildsystems/vcpkg.cmake -DVCPKG_OVERLAY_TRIPLETS=D:/a/dolfinx/dolfinx/vcpkg-triplets -DVCPKG_HOST_TRIPLET=x64-win-llvm -B build-dir -S .
+          cmake -DINSTALL_RUNTIME_DEPENDENCIES=ON -DCMAKE_TOOLCHAIN_FILE=$env:VCPKG_INSTALLATION_ROOT/scripts/buildsystems/vcpkg.cmake -DVCPKG_OVERLAY_TRIPLETS=${{ github.workspace }}/vcpkg-triplets -DVCPKG_HOST_TRIPLET=x64-win-llvm -B build-dir -S .
           cmake --build build-dir --config Release
-          cmake --install build-dir --config Release --prefix D:/a/dolfinx/basix-install
+          cmake --install build-dir --config Release --prefix ${{ github.workspace }}/basix-install
           echo "D:/a/dolfinx/basix-install/bin" | Out-File -Append -FilePath $env:GITHUB_PATH -Encoding utf8
 
       - name: Install build dependencies (workaround)
@@ -70,7 +70,7 @@ jobs:
       - name: Install Basix (Python)
         working-directory: basix/python
         run: |
-          python -m pip -v install --check-build-dependencies --no-build-isolation --no-cache-dir .[ci] --config-settings=cmake.args=-DBasix_DIR=D:/a/basix/install/lib/cmake/basix --config-settings=cmake.args=-DVCPKG_OVERLAY_TRIPLETS=D:/a/dolfinx/vcpkg-triplets --config-settings=cmake.args=-DVCPKG_HOST_TRIPLET=x64-win-llvm
+          python -m pip -v install --check-build-dependencies --no-build-isolation --no-cache-dir .[ci] --config-settings=cmake.args=-DBasix_DIR=D:/a/basix/install/lib/cmake/basix --config-settings=cmake.args=-DVCPKG_OVERLAY_TRIPLETS=${{ github.workspace }}/vcpkg-triplets --config-settings=cmake.args=-DVCPKG_HOST_TRIPLET=x64-win-llvm
 
       - name: Checkout FFCx
         uses: actions/checkout@v4