Revert "no need for a checkout for keepalive"

The workflow is failing to run. Does it actually need a checkout for
some reason?

This reverts commit 2b1bcac.

.github/workflows/keepalive.yml

@@ -10,6 +10,7 @@ jobs:
     name: Keepalive Workflow
     runs-on: ubuntu-latest
     steps:
+      - uses: actions/checkout@v4
       - uses: gautamkrishnar/keepalive-workflow@v2
         with:
           workflow_files: "update-mk-ca-bundle.yml, update-cacert-file.yml"