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/vtest.yml b/.github/workflows/vtest.yml
index 1dc216e..4cdbdce 100644
--- a/.github/workflows/vtest.yml
+++ b/.github/workflows/vtest.yml
@@ -11,6 +11,9 @@
 on:
   push:
 
+permissions:
+  contents: read
+
 jobs:
   # The generate-matrix job generates the build matrix using JSON output
   # generated by .github/matrix.py.