Use SRC_PATH instead of PLAN_CONTEXT/..
Use SRC_PATH instead of PLAN_CONTEXT/..

We were using `$PLAN_CONTEXT/..` to go to `/src` but habitat has an
environment to go there directly, let us use it! `SRC_PATH`

Signed-off-by: Salim Afiune <2b0aff3923bd0ab6bbf45ffb37e57dcea8e1b0b8@chef.io>

chef/chef-dk,chef/chef-dk,chef/chef-dk