Try to make debug workflow demand triggerable

The docs claim that by adding the right `on:` event, the job can be
manually triggered from the github UI.

https://docs.github.com/en/actions/learn-github-actions/events-that-trigger-workflows#workflow_dispatch

Tests show that it doesnt work, but I can leave it here for reference.

.github/workflows/debug.yml

@@ -1,10 +1,8 @@
 ---
 name: Debug
 
-# yamllint disable-line rule:truthy
 on:
-  push:
+  workflow_dispatch:
-  pull_request:
 
 jobs: