use default values for new SRC_DEV_ONLY, SRC_DEV_EXCEPT bash vars
Created by: shrouxm
fixes #14054 by supplying default values for the bash variables introduced by https://github.com/sourcegraph/sourcegraph/pull/14049
Created by: shrouxm
fixes #14054 by supplying default values for the bash variables introduced by https://github.com/sourcegraph/sourcegraph/pull/14049