Restrict job permissions in GitHub actions workflows

Restrict the GitHub token permissions only to the required ones, i.e.
just read-only access to the code.

This is done in order to reduce the potential harm in case of a
malicious pull request, see GitHub blog post at
https://securitylab.github.com/research/github-actions-preventing-pwn-requests/

Signed-off-by: naveen <172697+naveensrinivasan@users.noreply.github.com>

Closes #22574.

.github/workflows/code_checks.yml

@@ -9,6 +9,9 @@ on:
     branches:
       - master
 
+permissions:
+  contents: read
+
 jobs:
   check-unix:
     runs-on: ubuntu-20.04