Attempt to fix emscripten deployment (#414)

Author: mcbarton

Diff for: .github/workflows/deploy-pages.yml

@@ -185,8 +185,8 @@ jobs:
           micromamba activate xeus-lite-host
           python -m pip install jupyterlite-xeus
           jupyter lite build --XeusAddon.prefix=${{ env.PREFIX }} --output-dir dist
-          cp $PREFIX/bin/xcpp.data dist/extensions/@jupyterlite/xeus/static
-          cp $PREFIX/lib/libclangCppInterOp.so dist/extensions/@jupyterlite/xeus/static
+          cp ${{ env.PREFIX }}/bin/xcpp.data dist/extensions/@jupyterlite/xeus/static
+          cp ${{ env.PREFIX }}/lib/libclangCppInterOp.so dist/extensions/@jupyterlite/xeus/static
 
     - name: Upload artifact
       uses: actions/upload-pages-artifact@v3