ci: update workflow permissions for dependabot (#508)

Author: PierreJeanjacquot

.github/workflows/pr-test.yml

@@ -1,8 +1,12 @@
 name: PR test
-description: tests the PR
 
 on: [pull_request]
 
+permissions:
+  # Allows the workflow to write pull request comments (required for dependabot in docker-dry-run job)
+  # https://docs.github.com/en/code-security/dependabot/troubleshooting-dependabot/troubleshooting-dependabot-on-github-actions#changing-github_token-permissions
+  pull-requests: write
+
 concurrency:
   group: ${{ github.ref }}-pr-test
   cancel-in-progress: true