cascading-run: check the correct app attribute

Turns out that the `check_run` event does not have a top-level `app`
attribute...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

GitForWindowsHelper/index.js

@@ -66,7 +66,7 @@ module.exports = async function (context, req) {
     try {
         const { cascadingRuns, handlePush } = require('./cascading-runs.js')
         if (req.headers['x-github-event'] === 'check_run'
-            && req.body.app?.slug === 'gitforwindowshelper'
+            && req.body.check_run?.app?.slug === 'gitforwindowshelper'
             && req.body.repository.full_name === 'git-for-windows/git'
             && req.body.action === 'completed') return ok(await cascadingRuns(context, req))