Run Mayhem for API only when triggered manually

It currently always fails and we are not using it. We may use it in the
future though and the results can be interesting. So I'm keeping it but
it's not triggered on every pull request. How to trigger:

https://docs.github.com/en/actions/managing-workflow-runs/manually-running-a-workflow#running-a-workflow

.github/workflows/mapi.yml

@@ -1,5 +1,5 @@
 name: 'Mayhem for API'
-on: [push]
+on: workflow_dispatch
 jobs:
   test:
     if: ${{ github.repository_owner == 'openfoodfoundation' }}