ci: pin unpinned actions ref (#15551)

Fixes error found in the new zizmor release. Previously some of these
actions were exempt from the pinning. Just pin for consistency,
dependabot takes care anyway

See explanation in release notes:
* https://docs.zizmor.sh/release-notes/#changes

Author: rmuir

.github/workflows/auto-format.yml

@@ -79,7 +79,7 @@ jobs:
           fi
 
       - name: Checkout PR code
-        uses: actions/[email protected]
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
         with:
           repository: ${{ env.head_repo_full }}
           ref: ${{ env.head_ref }}