ci: declare workflow-level contents: read on 1 workflows

Pins the default GITHUB_TOKEN to contents: read on the workflows in
.github/workflows/ that don't call a GitHub API beyond the initial
checkout. The other workflows in this directory are left implicit
because they need write scopes that a maintainer is better placed
to declare.

Motivation: CVE-2025-30066 (March 2025 tj-actions/changed-files
compromise) exfiltrated GITHUB_TOKEN from workflow logs. Per-workflow
caps bound runtime authority irrespective of repo or org default,
give drift protection if the default ever widens, and are credited
per-file by the OpenSSF Scorecard Token-Permissions check.

YAML validated locally with yaml.safe_load.

Signed-off-by: Arpit Jain <arpitjain099@gmail.com>

Author: arpitjain099

.github/workflows/ci.nightly.yml

@@ -15,6 +15,9 @@ concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}-${{ github.ref != 'refs/heads/master' || github.run_number }}
   cancel-in-progress: ${{ startsWith(github.ref, 'refs/pull/') }}
 
+permissions:
+  contents: read
+
 jobs:
   test-nightly:
     # We do not run the full test suite on tags, because we already ran it on master before we made the release.