Allow manually running the changelog workflow

When the changelog generation failed, or when developers want to
regenerate it.

https://docs.github.com/en/actions/using-workflows/manually-running-a-workflow

.github/workflows/mkchangelog.yml

@@ -3,6 +3,7 @@ name: 📃 Changelog
 on:
   schedule:
     - cron: 0 0 * * 1
+  workflow_dispatch:
 
 jobs:
   generate-changelog: