* Use PR number directly in netlify github action
Looks like we can get the PR number for the workflow run that triggered
the workflow_run event, so there's no need for the massive faff we were
doing here.
* No need to create pr.json anymore
Some absolutely horrenous hacks to upload the context as an artifact
then download it, unzip it and set the PR number as a variable we can
use, because GitHub Actions just doesn't offer any other way of doing
this.
Maybe we'd be better off going back to Netlify...
So we don't need netflify to sit there building the develop branch
for no reason. Also it will comment with the URL rather than having
to dig it out of the checks.
Upload part not tested yet.
