Ensure new builds are triggered on pushes to the `main` branch

[punchagan]

With the current app settings, new builds are not triggered when a new commit is pushed to main. Pushes to PR branches do trigger a POST to the webhook, and current-bench ends up checking all the branches for updates, which ensures that the latest commit on main is benchmarked too. But, triggers to main don't directly trigger a new build.

Adding read permission on contents and enabling webhook triggers for the push event would fix this problem.

[punchagan]

The permissions have been changed on the production ocaml-benchmarks application, which would trigger emails being sent to existing users of the app.