Allowing pip-install to have access to environment variables

- Currently, `pip-install` does not have access to environment variables. 
- This PR is particularly useful when installing a pip package that requires an apt-package. For example, `scipy` requires `libatlas-dev` -- in this case, I can use `heroku-buildpack-multi` and `heroku-buildpack-apt`, and environment variable `BLAS=/app/.apt/usr/lib/libblas/libblas.a` would let the pip package `scipy` knows the location of the static library during `pip-install`.