infra/workflow: Declare limited permissions for regression

Note that this workflow currently imagines writing a comment.
It does not actually try to do so, and thus there is no need to ask
for permission to use `pull-requests: write`.

If at some point, someone decides to fix that feature, they
will need to add that permission.

OTOH, using a GitHub Step Summary instead does not require any permissions.

.github/workflows/regression.yml

@@ -6,6 +6,9 @@ on:
     branches:
       - main
 
+permissions:
+  contents: read
+
 jobs:
   test:
     runs-on: ubuntu-latest