Fix Netflify builds from fork PRs

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...

.github/workflows/layered-build.yaml

@@ -16,4 +16,16 @@ jobs:
                   path: element-web/webapp
                   # We'll only use this in a triggered job, then we're done with it
                   retention-days: 1
+            - uses: actions/github-script@v3.1.0
+              with:
+                script: |
+                    var fs = require('fs');
+                    fs.writeFileSync('${{github.workspace}}/context.json', JSON.stringify(context));
+            - name: Upload Context
+              uses: actions/upload-artifact@v2
+              with:
+                  name: context.json
+                  path: context.json
+                  # We'll only use this in a triggered job, then we're done with it
+                  retention-days: 1