don’t limit config vars to a subprocess

The current `compile` script limits config vars to a subprocess around `npm install`, but there are other commands in the `compile` script (such as npm prune) that are [executed outside of that context](https://github.com/heroku/heroku-buildpack-nodejs/blob/539550a90181c18c897a5208a31cf6509ba369f6/bin/compile#L69). This patch makes the more broadly available in the `compile` script.