Use hab for SVC_GROUP instead of root
Use hab for SVC_GROUP instead of root

`core/nginx` is currently compiled to use the `hab` user and group for
worker processes. By using `hab` for `SVC_GROUP`, we can ensure that
the workers have access to the files they need in the service
directory.

Signed-off-by: Christopher Maier <b4598f17a90ef80817bae8ad6c37249740caafb4@chef.io>

habitat-sh/habitat,nathenharvey/habitat,georgemarshall/habitat,georgemarshall/habitat,nathenharvey/habitat,georgemarshall/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,nathenharvey/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,nathenharvey/habitat,georgemarshall/habitat,georgemarshall/habitat,nathenharvey/habitat,rsertelon/habitat,georgemarshall/habitat,habitat-sh/habitat,georgemarshall/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,georgemarshall/habitat,rsertelon/habitat,habitat-sh/habitat,nathenharvey/habitat,rsertelon/habitat