ci: don't persist the checkout token

Nothing after checkout should need the GITHUB_TOKEN so persisting would only be
an extra risk.

Signed-off-by: Andrei Gherzan <andrei@gherzan.com>

.github/workflows/yocto-layer.yml

@@ -18,6 +18,7 @@ jobs:
         uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           fetch-depth: 0
+          persist-credentials: false
       - name: Check for Yocto relevant changes
         id: changed-files-specific
         uses: ./.github/actions/changed-files