workflow: Add top-level permissions

Set:
```
permissions:
  contents: read
```
as the default top-level permissions explicitly
to conform to recommended security practices e.g.
https://github.com/ossf/scorecard/blob/main/docs/checks.md#token-permissions

.github/workflows/payload-after-push.yaml

@@ -5,6 +5,9 @@ on:
       - main
   workflow_dispatch:
 
+permissions:
+  contents: read
+
 concurrency:
   group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}