Use PR number directly in netlify github action (#7561)

* 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

.github/workflows/layered-build.yaml

@@ -17,16 +17,4 @@ 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}}/pr.json', JSON.stringify(context.payload.pull_request));
-            - name: Upload PR Info
-              uses: actions/upload-artifact@v2
-              with:
-                  name: pr.json
-                  path: pr.json
-                  # We'll only use this in a triggered job, then we're done with it
-                  retention-days: 1