Update README.md

Author: mcbarton

README.md

@@ -102,7 +102,7 @@ To build Jupyter Lite with this kernel without creating a website you can execut
 micromamba create -n xeus-lite-host jupyterlite-core
 micromamba activate xeus-lite-host
 python -m pip install jupyterlite-xeus jupyter_server
-jupyter lite build --XeusAddon.prefix=${{ env.PREFIX }} --contents ../notebooks/xeus-cpp-lite-demo.ipynb
+jupyter lite build --XeusAddon.prefix=$PREFIX --contents ../notebooks/xeus-cpp-lite-demo.ipynb
 ```
 
 We now need to shift necessary files like `xcpp.data` which contains the binary representation of the file(s)