Added environment support to the install buildpack process

- If a custom buildpack is being used chances are it's because there is something unique about the environment
- The most standard annoyance in enterprise environments is the lack of direct outbound connectivity
- Proxy support needs to be propogated during the entire end to end build process.
- http_proxy and https_proxy can be added as environment variables
- Custom buildpacks can use the environment variables to be proxy aware
- install-buildpack is now proxy aware; it simply loads the environment variables from ENV_DIR