Added support for buildpacks' ENV_DIR.

flynn-receive sends a tar to Shelf containing for each env var a file with its name and value as its content.
This fetches that tar and extracts it to be used by the buildpack bin/compile.

* see: flynn/flynn-receive#7

Signed-off-by: vimes <joaomares@gmail.com>
Ditto. Only specify this if we actually have an env_dir.