Move executor shared secret in site config

Created by: eseliger

This makes it easier to change it and to configure executors for the first time. You just need to edit something in the UI, instead of adding a secret, expose that as an env var on frontend, and then restart it.

I think this closes https://github.com/sourcegraph/sourcegraph/issues/25233. There was a conversation going on in there if we should use a different approach, and I think ultimately we need to, but for now, I think this is a good quick win before we let customers run through this rather complex setup flow.