CI: Add `permissions` to GitHub Actions

This change locks down the permissions of the access token in GitHub Actions to
only allow reading the repository contents and nothing else.

see https://docs.github.com/en/actions/security-guides/automatic-token-authentication#permissions-for-the-github_token
diff --git a/.github/workflows/compliance.yml b/.github/workflows/compliance.yml
index 9f2bec2..fe6c271 100644
--- a/.github/workflows/compliance.yml
+++ b/.github/workflows/compliance.yml
@@ -5,6 +5,9 @@
   schedule:
     - cron: "0 0 * * 3"
 
+permissions:
+  contents: read
+
 jobs:
   h2spec:
     name: h2spec