env.deployment_status should be a job output

[rossjrw]

JamesIves/github-pages-deploy-action sets env.deployment_status and steps.[id].outputs.deployment-status to indicate whether the deployment succeeded / was skipped / failed etc. I use this to determine whether to leave a comment on the PR.

As of #97, everything that my action generates to use in its comment is also provided as an output so the user can write a comment with at least the same amount of information. The exception is github-pages-deploy-action's deployment_status, which is implicitly generated by that action and therefore never gets mentioned in my documentation, even though it's important.

Also, all of my outputs are exposed as step outputs and might well be accessible as env vars but I don't document that and I'm not going to make any guarantees about it either. The intended way to use them is via their step outputs. deployment_status cannot be accessed as a step output because, for a user of my action, there is no github-pages-deploy-action, so it's only available as an env var, and that's inconsistent and lame.

github-pages-deploy-action's deployment_status should be re-exported by my action as a step output so it can be accessed consistently.