Hi folks! I stumbled across an issue with the node package today and wanted to send a report before I forget.

npm assumes that the global prefix is a writeable folder. Operations like `npm link` will fail if it isn't. Right now our node package doesn't set a prefix, so it defaults to the package's directory in the store, which isn't good.

Maybe the solution is to select a folder inside the user's Guix profile (or perhaps in their XDG_CACHE_HOME, if any) and set that explicitly as the node global prefix using a profile hook.

In my case, I ran `npm config set prefix /home/ryan/.cache/npm` as a workaround.