ssbc: Simplify input to src exec

Created by: eseliger

This is the server-side counterpart to https://github.com/sourcegraph/src-cli/pull/798. I refactored the exec command in src-cli to be able to run runSteps() directly, which will help with the k8s experiments. As a neat side-effect, it became finally clear which config options we actually need and which we don't and where the client would be involved. The answer is: nowhere. So we can get rid of the duct tape env var and some more things passed down, like the sourcegraph version.

Test plan

Verified with src-cli build that execution still works.