Enable e2e GHA workflow to be executed manually (#650)

I need to debug the e2e GHA because it is failing.

Currently, this workflow is not triggered on changes which is fine, as
we do not want to start the e2e test on any change on GHAs.

However, it should be possible to trigger the workflow manually.

Author: frewilhelm

.github/workflows/dispatch-e2e.yaml

@@ -18,6 +18,8 @@ on:
   schedule:
       - cron: '0 6 * * *' # 6 AM UTC everyday for default branch
 
+  workflow_dispatch:
+
 permissions:
   contents: read # for actions/checkout to fetch code