benchmark: Adjust workflow permissions to allow it to write contents

This is needed so that it can add a comment on the commit that triggered
it.

.github/workflows/benchmark.yml

@@ -7,6 +7,10 @@ on:
  # Allows to run this workflow manually from the Actions tab
  workflow_dispatch:
 
+permissions:
+ # Contents permission needed to create a commit comment
+ contents: write
+
 jobs:
  benchmark:
  name: Run Schematron benchmark