Update permissions

This does not really change the default much but it's a decent practice
and makes the SSF Scorecard look better.

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

.github/workflows/conformance.yml

@@ -5,6 +5,9 @@ on:
   pull_request:
   workflow_dispatch:
 
+permissions:
+  contents: read
+
 name: Conformance test
 jobs:
   conformance: