bug: can't start enterprise dev environment on recent main

Created by: shrouxm

@keegancsmith I think this was due to https://github.com/sourcegraph/sourcegraph/pull/14049

If I enterprise/dev/start.sh I get:

./dev/start.sh: line 169: SRC_DEV_ONLY: unbound variable

but if I SRC_DEV_ONLY="" SRC_DEV_EXCEPT="" enterprise/dev/start.sh everything runs fine, so not blocking.