conf: Expose ExecutorsEnabled to frontend if executor.accessToken is set

Created by: efritz

Fixes https://github.com/sourcegraph/sourcegraph/issues/27866. Instead of creating a new flag to enable executors or document that you need to also enable another feature first to get this set up, we should just guard the executor-specific pages when the accessToken is set in the site-config.

Side-effects: We now need to set the executor.accessToken in dev our clones of dev-private when working locally as I've removed the fallback environment variable (that happens to be set in sg.conf.yaml for the executor service to read).