Only run deployability check on PRs and main branches

[shonfeder]

Currently, any branch pushed to ocurrent/opam-repo-ci triggers a deploy-ability check. Developers are probably aware of this and avoid using branches on the ocurrent fork, but it may be better if we avoid running the deploy-ability check on branches other than the "main" branches.

Pushes to live and live-web trigger actual deployments. The deploy-ability check needs to run before doing an actual deployment. Any PRs to master and actual commits on master also need to check for deploy-ability.