ci: add workflow_dispatch event

> You can now create workflows that are manually triggered with the new workflow_dispatch event.
> You will then see a 'Run workflow' button on the Actions tab, enabling you to easily trigger a run.
> You can choose which branch the workflow is run on.

https://github.blog/changelog/2020-07-06-github-actions-manual-triggers-with-workflow_dispatch/

.github/workflows/purge-readme-image-cache.yml

@@ -3,6 +3,7 @@ name: Purge image cache
 on:
   schedule:
     - cron: '54 18 * * */7'
+  workflow_dispatch:
 
 jobs:
   purge: