Use GitHub environmental files instead of using soon-to-be deprecated set-output

[sikai00]

What feature(s) would you like to see in RepoSense

Currently, our surge.yml uses set-output to extract and save the PR number for use as part of the surge.sh preview domain. However, GitHub will be deprecating set-output to "avoid untrusted logged data to use save-state and set-output workflow commands without the intention of the workflow author."

https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

This feature will be fully disabled by 31st May 2023, which is 2 months from now.

If possible, describe the solution

As described in the article, we can use GitHub environmental files available within an action to save the PR number instead.

https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#sending-values-to-the-pre-and-post-actions

If applicable, describe alternatives you've considered

Additional context

[dcshzj]

Setting this to high priority as it would break our deployment previews once it stops working.