ci: Security hardening for GitHub Actions

https://docs.github.com/en/actions/using-jobs/assigning-permissions-to-jobs

The idea is that the software supply chain relies on 3rd party actions
that could be compromised. Mitigate this risk by giving these actions
minimal rights to the repository. Here read-only access is good enough.

.github/workflows/ci.yaml

@@ -8,6 +8,9 @@ on:  # yamllint disable-line rule:truthy
     branches:
       - master
 
+permissions:
+  contents: read
+
 jobs:
   lint:
     name: Linters