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.
David Baker